11 Noises That Your Air Conditioner Shouldn’t Make – And What To Do About It

If you’re like most homeowners, you probably crank up your air conditioner during the summer months to keep your home cool and comfortable. But if you start noticing some strange noises coming from your AC unit, it may be time to call in a professional for repairs.

In this guide, we discuss 11 noises your AC should never make – and what to do about them.

So continue reading to stay cool!

AC technician inspecting the AC’s outdoor unit

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Humming Noises

Humming noises might be hard to notice since AC units have a slight hum when running. But if the humming gets louder and is a different tone than usual or more persistent, that’s when you should be worried.

What Causes It

A humming noise is usually caused by something calledelectrical resonance.” When the electrical current running through your AC unit starts to oscillate, it can create a humming noise. This can affect everything from portable air conditioners to central AC systems.

How to Fix It

The first thing you should do is check all the electrical connections on your outdoor unit to make sure they’re tight. But only service your AC unit when the power is off. You can turn it off at your breaker box (and sometimes with an outdoor breaker near your condenser).

If checking the electrical connections doesn’t work, you should call an HVAC technician for a service visit.

Clanking Noises

If you hear a hard clanking noise, that’s usually a sign that something has come loose inside your air conditioner. If your AC unit sounds like it is hitting two pieces of metal together, that’s certainly a sign you need to do something.

What Causes It

The most common reason for a clanking noise is that the fan blades have become loose and are hitting something else in the unit. Another possibility is that one of the metal panels inside your AC unit has become loose and is rattling around.

How to Fix It

If you hear a clanking noise, the first thing you should do is shut off your air conditioner at the thermostat. Then, look inside the outdoor unit to see if you can spot the problem. Sometimes a twig or other piece of debris can get lodged in the outdoor unit.

But if it looks like the fan blades are loose, don’t try to fix them yourself. Call an HVAC technician for a service visit – it takes skill and finesse to fix the blades, which you probably don’t have.

If you can’t see the problem, or if you’re not comfortable working on your AC unit, call an HVAC technician for a service visit.

Clicking Noises

technician checking the electrical wiring of an HVAC

Clicking noises are usually nothing to worry about – it’s just your AC unit turning on or off. But if a humming noise accompanies the clicking noise or it is clicking all the time, that could signal an electrical problem.

What Causes It

The clicking noise is just the sound of the relays inside your air conditioner switching on and off. But if a humming noise accompanies it, that is usually a sign of an electrical issue with the relay. And if it’s clicking all the time, the relay is stuck in the “on” position and attempting to switch “off.”

How to Fix It

If your AC unit is making a clicking noise accompanied by a humming noise, you should call an HVAC technician for a service visit. They can check the electrical connections and relays, find the problem, and replace the bad part.

If your AC unit is clicking all the time, you can try to reset it by turning off the power at the breaker box and waiting 30 seconds. Then turn it back on and see if that fixes the problem. If not, call an HVAC technician for a service visit.

Dripping Noises

If it sounds like water is dripping inside your AC unit, that’s usually a sign of a problem with the condensate drainage system.

What Causes It

The most common reason for a dripping noise is that the condensate pan is full, and the water is overflowing. The condensate pan is where the water from the evaporator coils drains. The water collects in the pan and drains out of your AC unit through a pipe.

Another possibility is that the drain line is clogged, and the water is backing up into the condensate pan.

How to Fix It

If you hear a dripping noise, you should first check the condensate pan inside your air handler to see if it’s full. If it is full of water, empty it. Then, check the condensate drain line for clogs. Use a pipe cleaner and a shop vacuum on the drain line discharge to unclog it.

If your AC unit is in your basement and the drain line empties outside, your AC unit will have a condensate pump. The pump moves water up, so it can drain outside. Check the condensate pump to see if it is in working condition.

If you suspect the condensate pump has failed or you can’t unclog the drain line, that’s a problem you’ll need an HVAC technician to fix.

Buzzing Noises

A buzzing noise from your AC unit is usually a sign of something loose or unbalanced inside the outdoor unit. But first, rule out bees and other home devices that can “buzz” first.

What Causes It

Just like clanking noises, loose fan blades can be the cause of buzzing noises coming out from your AC. Another possibility is that the fan itself is unbalanced. Either the fan chassis is loose, or the condenser pad isn’t leveled anymore (sometimes they sink in wet areas).

How to Fix It

If your AC unit is making a buzzing noise, you should first check the outdoor unit to see if there is anything loose. We’ll stress again to make sure the power is completely off before you attempt any DIY fixes.

If you see bolts holding the fan are loose or other parts causing the noise, try to tighten it or secure it, so it doesn’t move. Alternatively, if the condenser pad (the concrete pad your condenser sits on) is not leveled, you can try to make it leveled yourself.

But be warned, re-leveling a condenser pad is a lot of work. You’ll likely have to move the condenser, remove the pad, dig a hole, lay gravel, level it, then replace the pad and condenser. It’s a lot of heavy lifting that you shouldn’t (and probably can’t) do alone.

If you don’t see anything that looks like it could be causing the noise, get in touch with an HVAC technician.

Squealing Noises

a technician checking the outdoor unit of an AC

If your AC unit is making a squealing noise, that’s usually a sign that the bearings in one of the fans are going bad.

What Causes It

The most common reason for a squealing noise is that the bearings in one of the fans are going bad. The bearings help the fan rotate smoothly. Over time, they can wear out and start to make noise.

Another possibility is that the belt that connects the motor to the fan has become loose. The belt helps transfer power from the motor to the fan. If it’s loose, it can slip and squeal as it turns.

How to Fix It

If your AC unit makes a squealing noise, you should oil the bearings.

To oil the bearings, find the fan motor. There should be a small hole in the side of the housing. Use a few drops of oil (3-in-1 oil works well) and add it to the hole. Then, run the fan motor for a few minutes to distribute the oil.

Alternatively, if you suspect the squealing noise is coming from the belt, you can try to tighten it. The belt should have a tensioner that you can adjust.

If the noise persists, or you can’t find the source of the noise, get in touch with an HVAC technician. You may need a new belt or have an entirely different issue.

Rattling Noises

HVAC technicians assembling the outdoor unit of an AC

If your AC unit is making a rattling noise, it is yet another sound that means there’s something loose inside the outdoor unit.

What Causes It

The most common reason for a rattling noise is things like branches, twigs, acorns, leaves, etc., that have become trapped in your outdoor unit. And when the fan turns on, it makes these debris move around back and forth, generating a rattling sound.

It can also indicate loose screws or bolts inside the unit. Over time, screws and bolts can work themselves loose. When they do, they can rattle around and make noise.

Another cause of rattling noises is a failing contactor. The contactor is an electrical switch that turns the power to your AC unit on and off. When it fails, it can make a rattling noise.

How to Fix It

If your AC unit is making a rattling noise, check the outdoor unit for any debris causing the noise. If you see anything, remove it and see if the noise goes away.

If you don’t see any debris, check for loose screws or bolts. If you find any, try to tighten them – but if they’re too loose, replace them entirely.

Finally, if you suspect the noise is coming from the contactor, contact an HVAC technician. Unless you’re an experienced electrician or HVAC service person, we don’t recommend replacing this yourself.

Whistling Noises

If your AC unit is making a whistling noise, there’s likely an issue with the ductwork, refrigerant lines, or coils.

What Causes It

The most common reason for a whistling noise is a hole or gap in the ductwork. When air passes through the hole or gap, it can create a whistling sound. Another cause is small holes in your refrigerant lines or coils.

Your coils and refrigerant lines contain pressurized refrigerant that will escape through any holes and make a whistling noise. Leaks are always accompanied by decreased or no cooling from your AC unit, and the system will run more than normal.

How to Fix It

Check your ductwork for any holes or gaps if your AC unit is making a whistling noise. If you find any, seal them with HVAC “duct tape” or mastic sealant.

If you don’t see any holes or gaps in the ductwork, check the coils and refrigerant lines for leaks. You can use a refrigerant leak detector if you suspect leaks. If you find any leaks, get in touch with an HVAC technician. Only HVAC technicians with an EPA certification can repair your refrigerant leaks and recharge the system.

Hissing Noises

a technician using a manometer to check the AC’s refrigerant levels

If your AC unit is making a hissing noise, it’s likely due to a refrigerant leak. And it’s very unlikely to be an angry cat stuck in your ductwork (but you never know).

What Causes It

As we mentioned before, your coils and refrigerant lines contain pressurized refrigerant that will escape through any holes and make a hissing noise.

How to Fix It

The only way to fix a refrigerant leak is to get in touch with an HVAC technician who is EPA certified. They can find the leak, repair it (or replace coils if necessary), and recharge your AC unit with new refrigerant.

If you do have an angry cat in your HVAC system, we suggest catnip or canned tuna to lure it out.

Pulsating Noises

If your AC unit is making a pulsating noise, it’s likely due to an issue with the compressor.

What Causes It

The most common reason for a pulsating noise is the compressor failing. The compressor is responsible for moving refrigerant throughout your AC unit. When it fails, it can make a loud pulsating noise.

Another reason for a pulsating noise is that the compressor turns on and off too frequently. This can be caused by an overcharged system, a bad pressure switch, or a failing capacitor.

How to Fix It

If your AC unit is making a pulsating noise, get in touch with an HVAC technician as soon as possible. The compressor is a complex component of your AC unit and should only be repaired or replaced by an experienced technician.

If your compressor fails, most manufacturers offer lifetime warranties (or 20-30 years) on this component. So there’s a big chance you won’t have to pay to replace yours (besides install/labor costs).

Banging Noises

technician checking the fan of a central AC unit

If your AC unit makes a banging noise, it’s likely due to an issue with the blower motor or fan.

What Causes It

The most common reason for a consistent banging noise is that the blower motor or fan is coming loose. As you know by now, screws holding these components in place can loosen over time and cause vibration and noise.

Another reason for a banging noise is that the blower motor or fan is hitting something inside your AC unit. This can be caused by dirt, debris, or bent blades.

If you hear inconsistent banging noises, it is likely just your metal ductwork expanding and contracting. If the outside of the duct is warm and cool air from the AC unit rushes through, it could cause the aluminum to contract and become slightly concave or convex. This can result in a sudden banging noise.

How to Fix It

If your AC unit is making a banging noise, turn off the power to your AC unit. This will prevent further damage to the blower motor or fan.

Next, check the blower motor or fan to see if it’s loose. If it is, tighten any loose screws and bolts.

If the blower motor or fan isn’t loose, check for any dirt or debris that might be causing it to hit something inside the AC unit. Remove any dirt or debris and straighten any bent blades.

If you can’t figure out a fix, go ahead and call your local HVAC technician.

There’s not much you can do about inconsistent banging noises short of ripping out all your ductwork and replacing it with flexible ductwork.

Enjoy Some Peace and Quiet

No one wants their AC unit to make weird noises. Not only are they annoying, but they’re almost always a sign that something is wrong with your air conditioning system. To help prevent unwanted noises popping up with your AC unit, perform routine maintenance to keep it running smoothly – click here for more AC tips.

We hope this article helped you figure out what was causing your weird AC noises and now have some peace and quiet. If not, we recommend calling your local HVAC technician for assistance.

If this guide didn’t help you diagnose the problem you’re experiencing, check out our common air conditioner problems guide for help.

About The Author

Jonathon is a mechanical engineer with over ten years of experience in the HVAC industry. He has hands-on experience with all types of HVAC systems.

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