Air conditioners are not necessarily quiet, especially the outside AC unit. However, if the AC is making loud humming, grinding, buzzing, rattling, screeching, whistling, banging or squealing noises, there is something wrong which needs to be checked and fixed.
So, what causes an air conditioner to make these noises? And how can you tell where exactly the noise is coming from and how can you go ahead to stop it?
Humming, buzzing, grinding, screeching or rattling AC noises are caused by problems with motors, loose parts, broken fan blades, leaking refrigerant, bad contactors or capacitors, dirty filters and/coils, or blocked air registers. Loose thermostat wires or even dead batteries can also cause the problem.
If your inside AC unit is making rattling noises, it is highly likely that the blower fan blades are loose or even broken. It could also be caused by objects in direct contact with the unit so what when the blower kicks in, its vibrations causes the object to rattle.
When outside AC unit is making loud noises, you most likely have a bad compressor or condenser fan motor. It could also be caused by loose screws on the unit’s top grille or an open compressor relief valve due to pressure buildup.
Hissing sounds are often a sign of a leaking refrigerant. It could be inside or outside the house. However, if the hissing sound is sudden and so loud outside the house, it is highly likely that the pressure is so high that the compressor relief valve has popped up.
If you hear the sound of ice cracking in your AC, it is usually due to a frozen evaporator coil. AC evaporator coils freeze due to clogged air filters, low refrigerant levels, dirty coils, leaking or clogged ductwork and clogged up AC drain line.
Troubleshooting an Air Conditioner Making Noises
When your air conditioner is making weird noises, the first thing you should do is try to find out where the noise is coming from and what type of noise it is.
What I mean is understanding if the noises is coming from the outside unit (condenser) or whether it is coming from the inside unit. That way, it becomes very easy to troubleshoot and fix the problem.
In a nutshell, these are the different noises from air conditioners and what they mean:
- Banging noise – Problems with the compressor
- Clattering noises – loose or broken fan blades or vibrating objects close to the furnace
- Hissing noises – refrigerant leak or popped up compressor relief valve
- Whistling noises – dirty air filter or blocked registers
- Rattling noises – loose screws or broken fab blades
- Bubbling noise – clogged AC drain line or air inside refrigerant pipes
- Clanking noises – blower or condenser fan are out of balance
- Clicking noises – faulty thermostat
- Buzzing noises – dirty coils, bad capacitor or failing motor
- Squealing noises – blower/fan about to fail or broken fan belt
- Humming noises – faulty compressor motor or bad contactor
- Pulsating noises – problems with refrigerant lines
- Crackling noises – Frozen AC
As you can see, a noisy air conditioner can be caused by lots of things. Some of the problems are easy to fix while some will need to be fixed by a professional HVAC technician. In some cases however you may need to replace parts, some of which are quite expensive.
Let us look at the different ways of fixing an air conditioner making noises. I will start with noises from the outside unit and then tackle noises from the inside unit.
How to Fix AC Outside Unit Making Noises
If your air conditioner noises are coming from the outside/condenser unit, you most likely have a problem with your compressor or condenser fan. The fan is located at the top of the unit while the compressor is located inside the condenser coil at the bottom.
While cooling the house, the compressor sends hot and high-pressure refrigerant to the condenser coils for condensation to happen. The fan then pulls air from the surrounding and blasts it across the coils where it absorbs heat from the refrigerant and exits at the top of the unit.
As such, for cooling to happen, the fan must be spinning all the time. In order to spin, the fan needs a motor, which is prone to wear and tear and that could be the reason for the loud noises from the outside unit as metal grinds against metal.
The same fate can also befall the compressor motor. Parts inside the compressor can become loose or even break. When that happens, banging, grinding or even screeching noises can be heard from the outside unit.
Before you conclude that either the compressor or condenser fan motor has failed, there are other things which could also be the cause of the problem which you should check out first.
The following are some of the ways to fix an AC outside unit making noises:
1. Check if the Condenser Fan is Spinning Freely
You can see the condenser fan from the unit’s top grille. Is the fan spinning in the first place? If the fan is not spinning, it could be a problem to do with its motor or capacitor.
If it is not spinning but the unit is buzzing, grab a thin stick or screwdriver and attempt to manually spin the fan by turning it clockwise.
If the fan kicks in and starts spinning normally, you have a failed capacitor. On the other hand, if the fan doesn’t spin freely and comes to a stop, you most likely have a bad motor.
In a case where the fan is spinning but you can clearly hear the loud clanking noises, you most likely have a bad condenser fan motor. You may need to lubricate the motor and if that doesn’t work replace it altogether.
2. Check if Condenser Fan is Out of Balance
If your condenser fan is operating out of balance, it is bound to come into contact with the sides of the condenser unit and make a loud clattering sound. Condenser fan can operate out of balance for different reasons.
Usually, the fan will have several blades but I have seen instances where one blade has come off and fallen inside the condenser unit. The loss of the blade will shift the center of balance of the fan causing it to operate out of balance.
A loosely held fan will also be off balance. That happens when one or a few of the screws securing the fan assembly loosens.
By peeping through the top grill, you should be able to see if the fan is off balance and if it is making contact with anything else, and hence the cause of the noise. If that is the case, turn off the air conditioner and contact an HVAC technician.
3. Tighten the Top Grille Screws
I have seen this issue so many times. As you already know, the condenser unit’s top grille (which carries the condenser fan) is attached to the rest of the unit using screws. After years of usage, the screws will become loose due vibrations in the unit.
Loose screws also mean a loose AC top grille. As the condenser fan is spinning, the grill will vibrate as well as it bangs against the unit and that could be the reason for the clattering noises from your air conditioner’s outside unit.
The solution for this problem is an easy one. Just grab a screwdriver and tighten all the screws. That should stop the noise.
In some cases however, you will notice that there will be a gap between the unit and the grill that won’t close even when you tighten the screws, what you can do in that case is to loosen the screws a little, insert a coin or washer in the gap and then tighten the screws.
4. Inspect the Fan Blades for Cracks
A cracked fan blade can be the cause of the clanking noise from your AC. It doesn’t even need to be a big crack. A hairline crack, which is also quite hard to see can cause vibrations across the fan and your AC will therefore make those noises when it is running.
If the AC is still running, turn it off so that you can inspect the blades when they are still. Carefully check if one of the blades is cracked. You may need to remove the grill and inspect the blades closely.
If any of the blades is cracked then you will have to remove it and replace it.
5. Test the Contactor
If you can hear a buzzing noise from the outside air conditioning unit, a bad contactor could easily be the problem. The contactor is a magnetic switch controlled by the thermostat.
When the thermostat calls for cooling, the thermostat sends 24 volts of current to the contactor coil which is magnetized and then pulls in a plunger (located at the center of the contactor). When the plunger is pulled in, the usual 240 volts of power can then flow to the unit and that is how it starts.
After years of usage, the contacts on the contactor can get depleted or dirty. When that happens, the air conditioner will struggle to kick in and that could be the reason why the AC is making the buzzing sound.
To check if the contactor is the problem, remove the side panel of the condenser unit to reveal the electrical components of the unit. The contactor will be the small part with screws and wires connected to it.
Warning: Be careful not to touch any of the wires since the system is still powered and could badly shock you.
Use a screwdriver with an insulated handle to tap the plunger (at the middle of the contactor) and check if the buzzing sound will stop. By gently tapping on the plunger, you will be helping it make better contact with the circuit so that power can flow to the AC components freely.
There is also another way to check if a faulty contactor is the problem. Turn off power to the AC (through the breaker box next to the unit or inside the house) but turn the thermostat to “cool”. That will allow only 24 volts to be supplied to the contactor.
When the thermostat calls for cooling, the plunger should be pulled in. If it isn’t pulled in, it is bad and will need to be replaced. Fortunately, a contactor is cheap and not hard to replace.
6. Replace a Bad Capacitor
If the contactor is good, you could be dealing with a bad capacitor. The capacitor is the cylindrical or oval component next to the contactor with terminals and wires connected to its top. There could be one dual capacitor or 2 single capacitors.
The work of the capacitor is to power up the motor during startup, and also provide a consistent flow of current to both the compressor and fan motors. When the capacitor fails or is about to fail, the motors will struggle to start and that could be the reason for a buzzing AC unit.
Here is how to test and replace the capacitor:
- Make sure power to the AC is turned off. Turn off the thermostat as well.
- Discharge the capacitor to make it safe to handle. Use a screwdriver with an insulated handle to short the common and HERM terminal and then the common and FAN terminal.
- Test the capacitance. Set the multimeter to capacitance (MFD) and test the microfarads between the common and HERM terminals then common and FAN terminals. Compare it with the capacitance indicated on the capacitor.
- If the capacitance is low, the capacitor is bad and will need to be replaced. When buying a replacement capacitor, make sure you buy another one with the same capacitance and voltage as the bad one.
- Before removing the bad capacitor, make sure that you note which wire is connected where, because that is also how you need to connect them to the new capacitor. Taking a picture with your phone is a good idea.
For more information on how to test a capacitor check out this post.
7. Test the Compressor Motor and the Condenser Fan Motor
Sometimes it is not easy to tell whether the source of the noises from your outside AC unit is the compressor or condenser fan. One thing you can do is to disconnect either of the components from power supply and find out.
If the noises are especially accompanied by a scenario where the AC is not cooling the house, but the fan is spinning just fine, you most likely have a bad compressor.
I personally like to disconnect the condenser fan motor wires first. Again, make sure that the power to the unit is turned off.
From the top of the unit (through the grille), trace the wires from the fan to the capacitor and circuit board. Disconnect them and cover the naked ends using tape to prevent them making from contact with other parts of the unit.
With the wires to the condenser fan motor disconnected, turn on power to the AC. If you can no longer hear the noise, the fan motor was clearly the problems. On the other hand, if the noise persists, the compressor is bad.
Repairing an AC compressor is not easy. If it is the source of the noise, you will most likely need to replace it, which is not cheap. In most cases, replacing the entire condenser unit is the best alternative.
8. Lubricate the Fan Motor
If the condenser fan motor is the source of the noise, sometimes just lubricating it with oil is enough to stop the noise. You don’t always have to replace it.
Tip: If you have an old AC units that uses a fan belt, a broken belt could be the source of the squeaky noise from the outside unit. Replacing the belt is both cheap and easy and will surely stop the squealing.
The following is how to lubricate an air conditioner condenser fan motor and stop the noises:
- Turn off power to the AC.
- Remove the screws holding the top grille.
- Turn the grill (with the fan and motor assembly) upside down.
- Locate the ports on the side of the fan motor and fill them with lubricating oil.
- If there are no oil ports, slowly pour the oil on the motor rod and give it time to penetrate through the motor bearings.
- Put the grill back and secure it with screws.
- Turn on power to the AC and check if the noises have stopped.
- If the noise continue replace the motor.
9. Clean the Condenser Coil
As the condenser fan is pulling in air from the surrounding, it also pulls in debris which ends up clogging the fins surrounding the condenser coil. When that happens, air will not get to the coils causing heat to be trapped inside the condenser unit.
An increase in temperature of the refrigerant also results in an increase in pressure. The compressor is fitted with a pressure relief valve which will pop open once pressure inside the system exceeds the set value.
When that happens, a loud hissing sound will be heard from the outside AC unit. The relief valve can release pressure because of other AC problems but a dirty condenser coil is among the top.
Here is how to clean a condenser coil:
- Turn off power to the AC unit.
- Remove all the condenser unit side panels/covers.
- Use a brush with soft bristles to clean the coils.
- Spray the coil with a coil clean or a mixture of water and dish soap.
- Rinse it using water, starting from the top all the wat to the bottom.
- Put the side panels back and turn on the AC.
A detailed guide on how to clean the condenser can be found here.
10. Check the Base of the Condenser Coil
Is your outside AC unit laying on a plastic pad or slab that has the soil around it removed by erosion over time? That could be the source of your problem.
When the unit kicks in, the pad will start to vibrate and that could be the source of the noise. What you can do in that case is secure the pad from underneath using soil, wood, concrete or even rocks until it is stable.
11. Check the Thermostat
Are there clicking noises in quick succession coming from your outside AC unit? That could be caused by two problems:
- Loose thermostat wires
- Dead or dying thermostat batteries
Dash inside your house and remove the thermostat housing by pulling on tabs on the sides and then slide it out. Grab each wire one at a time and try to see if there is one of them which is loose. If there is, go ahead and tighten it.
Although not all thermostats use batteries, weak batteries could be the cause of the noise, which causes the unit to short-cycle due to lack of constant power supply. Replacing the batteries with new ones will fix the problem.
How to Fix an Inside AC Unit Making Noise
If you have a mini-splits air conditioner, one of the noises you are most likely to come across is rattling noises. Sometimes it can also be vibrations being transmitted through the walls.
The following are the different ways to fix noises from an AC indoor unit:
1. Tighten the Screws
The wall-mounted indoor unit of a mini-split system is has so many parts joined together using screws. When one or several of these screws loosen after years of usage, it causes the part being connected to vibrate and tap against other parts and hence the rattling noise.
To access the loose screws, you will first need to remove the air filters and the front panel. If the filters are dirty clean them as well. Depending on the type of your air conditioner, removing the panel will be different from other types of AC units so you will need to check out your manual.
Once the panel is out, start tightening the screws one at a time until the rattling stops.
2. Make sure the Filters are Properly Installed
As a start, make sure that the knobs at the top of the air filter are properly fixed. Another thing you can check is the air filter hooks. Ensure that they are hooked on the front AC cover.
Although these are problems you can easily fix on your own, if the rattling noise is because by problems with blower motor, you will need to call a technician to fix the problem.
3. Locate the Source of the Rattle
If you have a central air system, your indoor AC unit can rattle for so many different reasons. One of the first thing you should do is locate the source of the rattling noise.
Is there an object in direct contact with the furnace or blower compartment? If so, that could be the source of the rattle. As soon as the blower motor kicks in, the vibrations cause the object to start rattling.
Next, check if there is a piece of insulation that has become undone (check the ductwork as well). As you can imagine, the insulation will keep vibrating once the blower starts and hence the rattling.
4. Lubricate the Blower Motor
If your blower motor has not been lubricated for some time, the bearings could become dry resulting in squealing or grinding noises due to metal to metal contact. Lubricating the motor will fix the problem.
Loud clattering sounds from the indoor unit could also be a sign of a broken blower fan blade. If that is the case, you will need to replace it.
Here is how to lubricate an air conditioner blower motor:
- Turn off power to the air conditioner. There will be switch next to the furnace.
- Remove the blower compartment panel to access the blower.
- Disconnect the circuit control board and put it aside.
- Loosen and remove the bolts securing the blower assembly and pull it out of the compartment.
- Locate the 2 oil ports on the motor and pull them out using sharp objects.
- Fill the ports with lubricating oil and then put back the plugs.
- Put back the blower assembly inside the compartment and secure it with the bolts.
- Install the circuit control board back.
- Put back the access panel and turn on power to the AC.
Lubricating a blower motor is also not as easy as a condenser fan motor. Also, not all blower motors have oil ports. Check out this video for more details.
5. Replace the Air Filter
If there is a whistling noise inside your house especially close to the indoor unit, you most likely have a dirty filter. The air filter is located inside the return duct and if it is dirty, the blower will force air to flow through it and hence the whistling noise.
Replacing the air filter is a sure way to fix the problem. In case you don’t know where your AC filter is located and how to change it, check out this post.
6. Check if Evaporator Coil is Frozen
A whistling sound can also be as a result of a frozen AC evaporator coil. The restriction of air by the ice causes the whistling noise.
An AC frozen evaporator coil is caused by:
- Dirty air filter
- Low refrigerant levels
- Dirty coil
- Clogged or leaking ductwork
Although you can switch the AC and give the AC time to thaw, you will still need to fix the cause of the frozen coil.
Note: A frozen evaporator coil can also cause a cracking sound, like the one cause by ice as it cracks. To check if the coil cracked, remove the access panel and have a look. A frozen coil will also not cool the house properly.
7. Stop a Leaking Refrigerant
Can you hear a hissing sound even when your air conditioner is off? You most definitely have a leak in one of your AC components. The hissing sound is caused by the refrigerant leaking out.
Unfortunately, a refrigerant leak is not something you can fix on your own. You will need to bring a technician who will first locate the precise location of the leak and then fix it.
8. Unclog the AC Drain Line
Is there a bubbling noise coming from next to the indoor unit? Most likely you have a clogged AC condensate drain line.
There is a drip pan under the evaporator coil where the condensate drips into after air has been cooled. A drain pipe is then connected to the drip pan to drain the condensate outside.
The drain line can however get clogged (sometimes partially) and as the condensate tries to drain out cause the bubbling noise. Cleaning the condensate drain line can stop the noise.
I have written a separate article on how to clean the drain line. Read it here.
Bubbling noises from the air conditioner can also be caused by bubbles inside the refrigerant lines. If that is the issue and the AC is cooling just fine I would ignore the problem since trying to fix it is expensive as it has to be done by a technician.
9. Insulate Refrigerant Lines
The movement of the refrigerant through the copper pipes causes them to vibrate and that is why they are usually insulated using foam sleeves. If then there are pulsating noises inside your house, some of the insulations may have come off and the pipes are in direct contact with the wall.
Trace your refrigerant lines from the outside unit all the way to the house and insulate anywhere the insulation has come off using rubber or foam insulation. That should stop the noises.