Air conditioners are noisy pieces of equipment. They make separate and distinct noises when starting, running and stopping at the end of a cycle.
Many are normal AC noises – nothing to worry about. Those are listed later in the article.
Knowing what your air conditioner should sound like can prevent alarm or a needless repair call – and they start at about $75 minimum service call fee.
This post is about the air conditioner making strange noises such as a loud buzzing noise, pulsating low/quiet/loud/quiet, humming, hissing or gurgling. You might think that last one can’t be normal! But it usually is.
Sound Diagnostics is Tricky Business
At Pick HVAC, we are committed to providing you with accurate information. When it comes to an air conditioner making odd sounds, even pros will differ on what the cause likely is. Of course, none of us would claim to know for sure what any sound is without inspecting the unit.
Therefore, this post gives the most likely diagnoses for common AC sounds. We can be more certain about some sounds than others. Give the solutions a shot when they are DIY. Otherwise, it’s always safe to contact an HVAC repair service to check it out. If they tell you it’s an expensive repair or you’re not sure you believe them, get a second opinion.
Air Conditioner Noises & What They Mean
The best way to approach this is to list noises separately, give their most common causes and what can be done to solve the issue – if anything needs to be done.
Air Conditioner Banging or Rattling
Let’s start outside and move indoors in a minute. And we might as well get the worse case scenario out of the way.
If the condensing unit outside is banging or rattling, it likely means there is a broken part in the fan or the compressor. If it’s the compressor, that is not good news – a broken compressor is costly, and if the AC is 10+ years old, it might be best to put the $1,250-$2,500 toward a new unit.
Note that Amana and Goodman and some Daikin ACs have lifetime compressor warranties. If your AC is under warranty when the compressor fails, you’ll only pay for labor, which is about $300-$500.
If it’s the fan motor, the repair bill will be somewhere between $250 and $700 with $475 about average.
First, visually inspect the unit from the outside or take off the cover of the condensing unit to determine if there is debris, like a stick, inside that could be making the rattling noise by hitting the fan, for example.
If you don’t see anything obvious – or if the compressor is obviously leaking or damaged – get a few estimates on the repair. Then you can decide whether repair or replacement is the best decision given the age of the unit, your budget and how long you expect to live there.
Banging or Rattling Indoors – Is the ratting or banging noise coming from the air handler/furnace?
Then, the most common issues are a broken fan motor, a $300-$600 repair, or an imbalanced fan motor. Balancing a fan motor should only cost the amount of the service call. According to our AC Repair Cost Guide, service calls cost $75-$200 and cover one hour of labor.
Solution: We recommend calling a technician to look at it. Inspecting a broken or imbalanced blower motor while it is running can cause serious injury.
AC – Loud Buzzing Noise
A condenser unit buzzing can be caused by one of several issues.
1). A bad contactor or contactor switch will prevent power from flowing to the compressor to get it moving. When the power flows to the contractor but not through it, it will buzz loudly.
An AC humming noise can also be the result of a bad contactor – sort of a soft buzzing noise
Solution: We’ve addressed a bad contactor in another post. An HVAC technician will charge about $180 on average, though it could be a little lower or higher. If you’re handy, replacing a contactor might be something you can do yourself.
2). A bad capacitor can prevent the fan from starting. It can buzz too.
Solution: Use a stick – something that won’t conduct electricity – and push the fan blade to see if you can get it moving. If it powers up, the capacitor is bad. The cost is about the same for AC contractor replacement.
3). A bad compressor will make a loud buzzing noise. If it is getting power, but not functioning, a buzzing air conditioner is the result.
Solution: As we noted above, if the compressor has to be replaced, consider replacing the entire condensing unit. That might be a better way to spend $1,250-$2,500.
4). Vibrations. If there are loose parts in the unit or if the isolation feet on the compressor, parts used to prevent vibrations, these issues make a vibration noise that can sound like the air conditioner is buzzing.
Solution: It’s probably best to have the unit checked by a technician
5). Indoor AC unit buzzing because it is frozen. There are 9 reasons an AC unit can freeze in summer.
Solution: Our Air Conditioner Freezing Up? guide covers all the issues and how to solve them.
Air Conditioner Pulsating Noise
Is your unit making a pulsing or pulsating noise – loud/soft/loud/soft?
This rhythmic pulsating is sometimes called harmonics.
1). The cause is usually sound waves coming from the compressor’s movements not being insulated. Is the AC on a plastic pad? This is often the problem. The motion of the compressor transfers to the plastic and it pulsates and hums.
It could also be caused by the refrigerant line contacting the wall as it passes through. Normally the lines are insulated with sound-dampening material.
Solution: See if you can determine exactly where the sound is coming from. If it’s the pad, then a wood pad or platform will probably end the pulsating. If it is coming from where the refrigerant lines enter your home, then moving the lines away from the wall and adding foam or rubber insulation might stop the pulsing.
2). Is it more of a metallic whirring, loud and soft?
This is caused most often by a loose fan or fan blade that is wobbling and causing the strange noise.
Solution: Again, try to isolate exactly where the sound is coming from. Does the fan blade seem to be wobbling?
If you can’t see an obvious problem, contact an HVAC technician.
AC is Hissing – What’s the Problem?
A hissing AC is almost always caused by leaking refrigerant. One of the refrigerant lines could be damaged, perhaps by a lawnmower or weed trimmer. Or one of the connections could have come loose from age and deterioration.
Solution: It is important find and repair a leaking refrigerant line ASAP. Otherwise, you’ll soon have no air conditioning, and your house will get uncomfortably warm.
The cost to repair a refrigerant leak ranges from about $200 for a minor leak and getting the proper charge back into the system. A major refrigerant leak requiring all new refrigerant costs up to $1,500, especially when a refrigerant line must be replaced and refrigerant added to the system.
Loud Squeaking or Squealing AC
When motor bearings go bad, they squeal, squeak or might make a grinding sound.
Solution: If the sound is outside, it is likely the bearing in the fan motor of the condensing unit. If indoors, it is probably the bearing in the blower motor. A DIY fix is possible if you’re really handy and follow instructions closely – while taking safety precautions when working with electrical equipment.
Otherwise, get estimates for AC repair from several contractors in your area.
Inside Air Handler is Whistling
This is caused by air traveling around the air handler or furnace cover. The most likely cause is a dirty air filter. When air can’t get through the filter, the strong blower motor will do its best to pull air into the unit.
It will cause a whistling sound as air passes through cracks and crevices.
Solution: Check and change the filter if necessary.
A leaky duct is another cause of the AC whistling. You’ll hear this near the air handler or furnace, wherever your blower motor is located.
Solution: Leaking ductwork causes air loss – and that costs you money when cooled or heated air leaks and never gets to your living spaces.
All ductwork should be sealed. If it is in space that isn’t heated or cooled, the ducts should also be wrapped and insulated.
Normal AC Noise
Before we discuss noises that are not a concern, here’s an important note:
If you are familiar with your HVAC system and know how it should sound, if a new sound develops, it likely means a problem. You should call a technician to look at it.
Here are a few sounds you might hear – and are completely typical.
Bubbling or Gurgling Air Conditioner Sound
Refrigerant cycles between the indoor and outdoor coil, pushed through the system by the compressor.
When refrigerant enters the coil, it can make a gurgling sound.
A Loud Click, Shudder or Clunk in the AC
When a cooling cycle stops, the compressor shuts off. There might be a pressure difference between the compressor and a refrigerant line. This pressure difference can cause a loud click, clunk or shudder of the unit. Nothing to worry about.
Humming – But Soft Humming
Motors working as they should emit a gentle hum as their parts whir around.
As Inspectapedia says, “Some compressors emit a high pitched noise during normal operation or just at startup. This noise should be brief and just at start-up, and will probably have always been present on the system if it’s normal.” If it’s a new sound, it should be checked out.
To Tech or Not to Tech?
If you’re not sure whether a sound is normal or indicates the need for service, it makes sense to contact an HVAC tech – a heating and cooling company.
We provide a free service through which you can get estimates from several local HVAC companies. They’ll send out a tech to answer your question, diagnose the problem and give you a no-cost, no-obligation written estimate. It’s that easy. The HVAC technicians are pre-screened for experience, and they are licensed and insured for your protection.