Photograph of an old burned-out air conditioning compressor unit HVAC Noise Descriptions & Recordings
Banging, bearing screeching, buzzing sounds, other noises.


InspectAPedia tolerates no conflicts of interest. We have no relationship with advertisers, products, or services discussed at this website.

Air conditioner or heat pump noise descriptions mapped into problem diagnosis & cure:

We have grouped HVAC system noises into groups by noise type and some alphabetic order. We start here with HVAC NOISE group 1, banging, bearings, buzzing sounds. The article includes a complete catalog of all HVAC system noises.

This air conditioning repair article discusses the diagnosis and repair of air conditioning compressor noises which range in importance from normal (if annoying squeaks and squeals, to rattling loose bolts and hardware, to costly compressor damage indicating air conditioning compressor or A/C compressors at or near end of their life.

We also provide a MASTER INDEX to this topic, or you can try the page top or bottom SEARCH BOX as a quick way to find information you need.

Catalog of AIr Conditioner or Heat Pump Noises by Description of the Sound

Here is an index to all of the diagnostic sounds or noises you may hear at an air conditioner or heat pump system either outdoors at the compressor/condenser unit or indoors at the air handler / blower unit or perhaps in the ductwork itself.

Keep in mind that some of these sounds may occur together and also that people may describe the same sound using different words.

One person's banging noise may be thumps or clunks to another while hissing and screeching and screaming may also be mixed-up together.

This means you may want to read about the causes and cures listed for several of the HVAC system noise articles listed here.

Index to Text Descriptions of A/C or heat pump sounds & possible causes / cures

Index to Recordings of A/C or Heat Pump Sounds & Noises: Submit, Listen-to, or Comment on These Sounds

Intermittent buzzing sound / video

This brief video contributed by one of our readers demonstrates start-up troubles and characteristic buzzing at the outdoor fan-compressor unit. The unit is unable to start.

Some guesses at what the noise means are given below.

Other videos: VIDEO GUIDES at

Squealing, clanking compressor noises

Video of noisy squealing clanking A/C compressor - condenser unit - 6 Sept 2013

This brief video contributed by one of our readers (L.M.) demonstrates start-up troubles and loud continuous squealing, clanking, and what I take to be some humming at the outdoor fan-compressor unit. The unit is unable to start and run normally.

A/C Compressor Noise Description:

Twice in the last month in a half, we’ve had this squealing metallic noise coming from the exterior condenser.  On both occasions the AC was running fine all day, then about 930 or 10 PM it starts making this noise.  In both cases it eventually stopped making the noises on its own.  It makes the noise only while AC is running.  Fan only is OK.

Some guesses at what the noise means are given below at Squealing air conditioner and heat pump equipment, diagnosis

HVAC Noise Group 1 Banging

Banging Sounds from A/C Compressor or Heat Pump, Frequent Compressor Replacements?

Common sources of loud banging, clanging or clanking noises at an air conditioner or heat pump include the following:

Watch out: you should immediately turn off equipment making these noises. Moving parts can cut wires risking fire or shock, or a loose fan can cut a refrigerant line or cause other more costly damage to the system. Always call for repair by a trained HVAC service technician.

Reader Question: banging sounds from the compressor

OK. I just reread the instructions and it looks like I can ask a question. In the past 5 years we have replaced 4 compressors. We just replaced the last one and the fan motor. Every once in a while, the unit would make a repeated really loud banging sound as though some metal piece in the fan was whacking another piece over and over. Today I watched the clock

It is almost at every hour, but not at the same time. Each time the noise lasts 3-4 minutes. The A/C company and their electrician have no clue what this is. Also, the unit often trips the circuit breaker. Not necessarily when this noise comes on. We are incredibly frustrated. Any ideas as what could be causing this? Thank you. - Dina

Question: banging compressor sounds

(Sept 29, 2015) Barb said:
We have Hyundai multi split ac. About 7 years old or more - not sure as it came with the house. Unit was off but suddenly started making loud banging noises.

Reply: common sources of loud BANG noises in air conditioning or heat pump equipment

Dina, my OPINION is that if you are replacing compressors 4 times in 5 years something is drastically wrong. I can't guess whether the problem is an innate system design pressure, poor or damaged equipment being installed, or installation errors.

But I share your frustration. I think it's time to talk candidly (but without threats or screaming) with the A/C company's service manager and to ask that they send someone who is trained and experienced to help diagnose this problem. And when you're told what problem was diagnosed, if you don't understand the explanation, ask for more help with understanding it until it makes sense.

A fan whacking something is a loose fan blade or broken fan shaft or bent screening around the fan or a stick or junk in the unit - any of which is a serious problem for the fan. If your service tech left the system like that s/he did not do such a hot job.

A unit that trips the circuit breaker is probably drawing high current - which, if the proper breaker has been installed, points to a problem with the wiring or more often the compressor motor on that circuit. A failing compressor motor draws high amps.

I'd also suggest checking out the voltage being provided to the system to be sure that there is not a low-voltage problem from your power source.


Thank you so much. Here's the problem. We just changed service companies , because we were not satisfied that the old one was doing a good job. This one decided that the compressor and then the fan needed replacing. They've had two sets of technicians out here and neither could figure out the problem. There is nothing hitting the fan (so they say). The problem is that they are never here when the sound happens, though

I did record the sound for them. They said it was nothing like anything they've heard before. They are coming out this morning again and we will see what they can find. Thank you for your "opinion". - Dina

The tech was here again. He noticed that the last guy had installed a booster. He disconnected it to see if it helps. Meanwhile, I'm wondering if the noise has something to do with the unit working harder in the hot weather because I hadn't noticed the noise at all today and it's much more comfortable outside. I guess I won't know until it turns really hot again. - Dina

Even with the booster disconnected, it's still making that banging metal sound. - Dina


Dina if the banging clanging is a compressor noise see the article titled NOISES, COMPRESSOR CONDENSER

Question: banging noises from heat pump when heat is running

Ron said:
My Heat Pump makes a constant banging sound only when the Heat is running. The video shows the fan moving oddly... but to the naked eye it seems fine. Any ideas?

Here is a video of the "banging" hard-starting fan on this outdoor compressor/condenser unit.



Ordinarily I'd say that to find the cause of ticking, rattling or banging at the fan component of a compressor/condenser you want to look for a loose fan bearing or bent blade or something obstructing the fan.

But after checking the video found in your banging compressor noise video (above) I see that the motor is having trouble starting and is cycling at start-up.

Possibly the the fan has a bad start/run capacitor. Try replacing that and let me know what happens.


Below is a video excerpt adapted from Ron's "banging compressor" sound video.

Banging or Hard Starting Compressor Condenser Unit Video


Banging Noises at the Air Handler or in Duct Systems

A loud BANG or THUD or CLUNK heard when the heating or air conditioning blower fan starts or stops is often due to inadequate return air, to overpressureizing of the supply plenum at fan start-up, or it may occur where large areas of un-supported sheet metal are used in the duct system or air supply or return plenums.

The problem may be more acute where a multiple speed or high speed blower fan is in use.

Question: furnace sucks in metal return air panels when it starts

2016/10/25 curt said:

I've installed a new 2 stage variable speed furnace in my basement, the air return is above it in one central register on the main floor about 30 x 12.

I have found that when the furnace starts it sucks in the metal air return panels in the basement that lead into the back of the furnace. I added another heating vent to the system into the basement, but have not increaased the air return yet. I have also turned the down speed of the fan to the lowest it will go, so now it ramps up slower. those 2 items have helped a lot.

However I need as well to leave the front of the furnace door open for additional air, otherwise it still sucks in the metal return. I was told to put bracking inside the return, which is an option, another option I thought would be to add another return to the other side of the furnce with another opening and filter.

A furnace man came out and said that would work, but wanted some others ideas and opinions. Thank you in advance, email to

This question was posted originally at UNDERSIZED RETURN DUCTS

Reply: solutions for start-up / shut-down BANG noise at metal duct work or air handler panels


I've seen and heard metal panel bangs, clunks, and thunks that can be a startling heating system noise easily traced to just what you observed. This loud BANG, described by some as a CLUNK or THUNK or even a WHOMP sound, can occur at both start-up and shut down of air handling heating and cooling systems.

What Causes the BANG at Blower Fan Start-Up or Shut-Down?

When the fan starts up the system is at least initially a bit air-starved creating a vacuum on the return side of the system. A powerful fan can also cause the same noise by bulging out rather than in sheet metal components on the supply side of the heating or cooling system.

Once the sheet metal has been bulged out by the pressure of supply air, when the blower fan stops we may hear a second BANG or CLUNK noise. In fact at a cabin we recently rented at the Sleeping Giant campground the shut-down noise seemed louder than the start-up noise, probably because it was not partly obscured by the noise of the blower fan itself.

How to Fix Start-up or Shut-down Air Handler BANGING

Other furnace or air conditioning duct noises such as hissing and howling can be from air velocity combined with duct shape, size, bends, or obstuctions, or from leaks in the duct system. For those see

A Simple DIY test for Inadequate Return Air

When you have to leave the a blower compartment door open then it's a pretty good indicator that the system needs more return air. Another similar test I make is to wait until the blower has been on for 5 minutes or more, then open the air handler blower compartment door about 6" and let go. If the door SLAMS shut with a powerful BANG then the system is probably air starved.

Bearing Noises in HVAC equipment

Reader Question:

Sounds like very loud bering noise on start up and shut down from a 1 year old heat pump compressor - Ray 12/5/11


Ray, a bearing noise, often a horrible clanking, banging, or rattling, is generated by a wobbling of a rotating shaft that is no longer evenly supported by bearings that are intended to permit it to rotate with minimal friction. A bad motor bearing can also cause a squealing or screeching noise, and if the bearing and motor shaft actually begin to bind you may hear a humming or buzzing from the electric motor.

If you hear bearing noises from an A/C compressor motor I suspect that the motor is failing, or that a similar noise is being made by damaged refrigerant valves in the compressor head. But first check that the motor mounts are secure. Sometimes a motor can make a clank or thunk at startup because one or more of its mounts has come loose.

Watch out: Shut off any electric motor that is screeching, humming, or running as described above. Then call for repair.

Buzzing Sounds at Air Conditioning or Heat Pump Equipment

This page addresses buzzing or vibration noises at air conditioners or heat pumps.

For other types of equipment buzzing and vibration sound complaints see

Common sources of buzzing noises or sounds at the air conditioner / heat pump unit or other equipment in a building

  1. Loose or missing isolation feet on the compressor motor itself or on the whole compressor/condenser unit.

    Check that both the motor and the whole outdoor unit a level and all feet are pressing on the support pad. You may need help from a trained service tech to remove the coer and then to inspect the interior of a compressor/condenser unit to see the isolation pads under the compressor motor.
  2. Bad relay control switch or contactor relay: Bad or failing compressor motor relay switch or bad fan contactor relay switch may buzz or hum.

    These are usually low-voltage (e.g. 24VAC) controls that are used to switch on the higher-voltage-operated motors of the compressor and fan.
  3. Loose components, screws, cabinetry, grilles, on the compressor/condenser unit outdoors or on the air handler indoors; also loose or poorly supported refrigeration piping or electrical wiring conduit that may contact and buzz against another surface as the equipment vibrates
  4. Fan motor failures: Bad or failing condenser fan motor on the compressor/condenser unit may buzz or hum and may fail to start.

    Similarly a bad fan motor in the air handler unit or blower unit can hum or buzz. Remeber to check the fan itself for mechanical damage, wobbling, bent blades as those can cause vibration that first shows up as noise but ultimately damages the motor bearings.
  5. Compressor motor failures: A compressor motor that tries to start but cannot get running may buzz or hum for a few seconds before its overload switch shuts down the motor.

    Low voltage or a bad start/run capacitor can also prevent an electric motor from starting successfully.
    Don't rush to replace a compressor motor when you hear buzzing noises as a mis-diagnosis can lead to an expensive and incorrect repair. Check other components in the system with care.
  6. Refrigerant leak or bad TEV: Refrigeration leak or low refrigeration charge causing icing anywhere in the system; Ice-blockage of a themostatic expansion valve (TEV) or electronic expansion valve (EEV) can cause buzzing noises as refrigerant passes through the device.
  7. Bad or clogged filter dryer canister at the HVACR unit
  8. Improper voltage, possibly low voltage, causing motors to have trouble starting: motors or contactors may hum or buzz. A bad start/run capacitor can also leave a motor buzzing but more often humming.

    Voltage drops or transients can cause motors to have trouble starting but may not be present when the service tech examines the system. Monitoring voltage levels may be necessary to find intermittent motor start or motor buzzing problems.
  9. Defective fuse or circuit breaker at the service switch or power control for the unit; Bad, defective, loose electrical connections causing arcing, humming, buzzing, crackling sounds, possibly smoke or odors as well.

    Watch out: corroded aluminum feeder wires, loose electrical connections, wet, rusted or damaged circuit breakers, improper fusing or similar electrical defects at the fuse or local service switch for an air conditioner or heat pump may cause worse than hissing or buzzing, risking as well an electrical shock or fire hazard.
  10. Loose electrical connections inside the equipment. A loose clip-on connector bringing power to a start/run capacitor or to a control relay can cause buzzing sounds in the equipment.
  11. Normal buzzing at the reversing valve: If your unit is a heat pump then check the solenoid coil switch that operates the reversing valve on your system - really ask your service tech to make these checks.

    For some heat pump systems the reversing valve solenoid may buzz for a time when the system stops operating - pressures are equalizing. It's normal.
  12. Air Handler Noises: AIR HANDLER / BLOWER NOISES for a discussion of noisy furnace or air conditioner air handlers or ductwork noises such as air leaks, clicking, thumping or banging duct noises, fan noises and vibration dampener noises or rattling at fan motors
  13. Duct System Noises: DUCT SYSTEM NOISES rattling & buzzing from loose components, if the noise is coming from or being transmitted by ductwork.
  14. More buzzing noises from heating equipment like furnaces, boilers, water heaters, are described at HEATING SYSTEM BUZZING NOISE and at OIL LINE BUZZ & VIBRATION CURE
  15. More buzzing noises from a water heater are at NOISE, WATER HEATER
  16. More buzzing noises from a water pump are at WATER PRESSURE SWITCH NOISES

Diagnostic tips for a buzzing air conditioner or heat pump

Reader Question: loud buzzing noise from compressor unit

I had problems with my central unit freezing up and dripping water on my fan motor.. decided to have a technician change out my coil inside.. when he left my central unit outside starting making noises i never heard before.

My air conditioner kicks on inside first then after a couple of minutes it tries to kick on outside , it makes a loud noise then it runs finally and quiet like a central unit should run outside.. what is causing this all of a sudden, never had problems until he changed my coils out..not calling him back to my house. Bruce McCann


HVAC compressor buzzing video: the HVAC noise diagnosis introduction near the top of this page includes a video example of brief buzzing at the fan/compressor outdoor unit along with some possible explanations. Below we discuss several A/C or heat pump compressor noise observations and possible explanations.

Bruce, I am not clear on why a coil freeze-up indoors would lead to changing the outside coil - maybe low refrigerant or improper high side vs low side pressures? In any case, I would indeed give the service tech a call, tell him or her what you are observing, and give the technician an opportunity to return, see what's wrong, and fix it.

Most technicians want their work to be correct and successful. Let us know what you're told - it will surely help other readers.

Question: can you tell us what the attached air conditioner buzzing clanking sound recording means?

Attached is a few seconds of the noise made by our Kenmore air conditioner/heat pump, when the propellers start turning, about 10 seconds into the recording. It was much worse in the winter with the heat pump, so we heated with oil only.

It was so bad, the house rattled inside. The compressor and motor were replaced a few months ago, but the noise persisted and the maintenance company gave up after eight visits. When the weather got hot, we decided to give the air conditioner a try.

The air conditioner was working without us noticing any noise but now we can hear it, as you can tell from the recording. What can we do? Your help would be greatly appreciated. - L.G.

Recording of Kenmore Air Conditioner Noise  - buzz, rattle, hard-start


Honestly, I'm not sure, but the sound we heard in your recording sounded like a mix of buzzing and rattling. I've heard sounds similar to this from both a failing crankshaft in a motor and from an air conditioner compressor that was liquid slugged with refrigerant due either to an improper charge or a malfunctioning refrigerant metering device.

If that's right we may be hearing a hard-starting compressor.

I have two suggestions: a technician, using a mechanic's stethoscope, ought to be able to pinpoint this noise to the offending part or motor. If the noise is traced to the compressor motor I suspect my guess above is not far off. It is possible that once the system has been running for enough time the noise may fade as the compressor has pushed the refrigerant out to the high side of the system.

Watch out: liquid refrigerant slugging of a compressor motor is a problem that occurs on the low pressure side of the system where the equipment is designed only to move refrigerant as a gas, not a liquid - liquid slugging of refrigerant at the compressor motor is likely to destroy it. See REFRIGERANT FLOODBACK, LIQUID SLUGGING

I hope you've already eliminated more obvious external rattles such as a bad bearing or motor shaft or fan blade ticking against screen or internal component.

At Recordings of A/C or heat pump sounds & noises we have introduced a new HVAC diagnostic feature (thanks to you) where we posted this air conditioner sound recording to permit other readers or experts to offer comments.

Question: buzzing from furnace fan compartment, fuse blows

(Apr 2, 2014) Sandor Lipschultz said:

40 y/o Payne furnace: after burners come on, buzzing sound from fan compartment for 20-30 sec. and then burners go off; on/off furnace switch fuse blows

Reply: check for a bad relay or contactor used to control HVAC motors

Buzzing is often a bad relay switch. Sounds as if there may be a failed motor or control. An overcurrent when a motor can't start-up can be the cause of blown fuses.

Occasionaly I also find buzzing at the low-voltage thermostat.

Question: Buzzing sounds from air conditioner

My outside fan motor is running but the unit makes a loud buzzing noise every few minutes or so, the motor was replaced last spring. I sprayed the motor shaft with wd40 just to get me by till I can get another fan motor if that is needed. I do not know if the capacitor was replaced or not. the loud buzz only lasts for a few seconds then stops, I just turned the unit back on, Any ideas on what I can do or a place to find a motor?- Rodney Looney

Compressor started producing a buzzer noise which lasted about 4 seconds then stopped and repeated again. The time between buzzes varied from a 10 to 20 seconds. The noise reminded me of the sound you hear on the radio when they do there check of the emergency broadcast system. - Jim

I have a Trane 4 ton gas pack that is making a intermittent buzzing noise when the compressor operates. We are located in Phoenix, AZ and the noise seems to have started this season when temps heated to 100 and above. Naturally, I'm concerned that this might be compressor noise since the unit was manufactured and installed before the AC season in 1994. Thanks for any suggestions you can offer. Doug

More info:
The unit is on the roof of a flat roof house and the noise is most noticeable when standing below the unit. The noise is a higher frequency buzzing that could be a panel or other object vibrating close to the frequency of the compressor. To my knowledge, I have not been on the roof when the noise occurred, or if it did, I didn't hear it with all the other noise. It seems to occur at various combinations of temperatures.

Under some conditions, there is no noise, but other times, such as during a long run when we lower the temp after the sun sets, the noise may start several minutes into the cycle. A few times, I noticed noise soon after starting that only lasted for 20 seconds or so. We run the indoor temps between 85+ and 81 over a 24 hr period using a programmable thermostat.

I have banged and thumped on the case, removed the outdoor fan grill and looked for loose fasteners within the compressor/condenser portion of the case and checked all of the fasteners on the outside of the case to make sure they were tight. I put card-stock shims between joints of panels, etc to prevent any vibration that might occur. So far, nothing has eliminated the noise that seems to occur only under certain combinations of outdoor/indoor conditions.

The noise pattern may be different from when I first noticed it. I checked the pressures, for the first time since we got the house 8 years ago and and added 4 lbs of R-22 to get it back to normal operating conditions at 95F. The performance isn't noticeably different, but the noise pattern may have changed somewhat, while the sound is about the same, when it does occur.

Last winter I removed the top of the unit to oil the indoor fan motor. That is the first time it has been removed since we owned the house. As far as I know, everything was reassembled correctly. My experience is limited to light to medium maintenance of only my own auto/home AC systems for the past 4 decades. I installed a quick start system about 5 years ago on general principals because one worked wonders on similar but smaller gaspack in another part of the house. - Doug

Reply: Buzzing Sounds at an A/C or Heat Pump System May Mean Failing Electrical Parts

Rodney: if you need a new condenser fan motor you should be able to match the original by taking the old one to your local HVAC supplier, or you can probably find a replacement online. I'd be sure you note the motor information and specs from it's data tags or just bring in the old motor to your supplier.

Watch out - don't keep replacing motors unnecessarily. A bad fan motor contactor relay or even a loose wire could also cause buzzing in the condenser/compressor unit outdoors. .

Jim: sounds like your compressor is having trouble starting up? Or does it buzz but run ok otherwise?


buzzing sounds from an air conditioning component might be a failing electrical component and could involve even a fire risk; I'd try to track down more carefully where the buzzing is occurring. A burned compressor contactor relay, for example, might buzz.

Noise diagnosis is tricky in part because there is some subjectivity in how people describe sounds (buzzing vs humming for example). Taking care to not get whacked by a moving part like a compressor fan, one can sometimes use a mechanic's stethoscope to get right to the source of a sound from mechanical equipment.

Follow up:

Thanks for your suggestions. I'm relieved that you didn't point to the compressor as a typical source for noise like mine, but I know I can't rule it out yet. I took a dowel rod up to listen to for any noises, but at 94 deg, it didn't make any abnormal sounds (my wife confirms).

I did notice that the top of the condensor at one edge was vibrating and a potential source of noise. I'll check it later. I opened the control compartment and all seemed normal and no loose components there.

I would like to be able to listen to the compressor, but there's no way without dueling with the outdoor fan and I would likely lose. I'll let you know if I ever come to any conclusions.

Follow up:

A while ago, I wrote about intermittent noise from my roof mounted gas pack. After spending a lot of time on the roof and poking around inside the compressor compartment, I am reasonably certain the noise is coming from the compressor itself. I checked the compressor mounts and they are solid and there are no loose parts to vibrate and cause noise.

In the process of pushing the compressor around to see if the mounts were loose, I heard the compressor hit the side of the shell.

I was looking around another site and the owner mentioned that the compressor suspension mechanism can break and cause noise that sounds like a bad bearing.

A friend of mine knows a person in the refrigeration business and after my friend described the problem, he said it sounded like a broken spring in the compressor suspension.

I guess I am going to have to replace the compressor before the start of the next season, if I make it through this one.
Hopefully I will replace it before there is a burnout.

Would you recommend installing a filter/dryer even if there is no hard failure? I don't believe Trane installs them in new-build units and there is certainly none in the compressor bay. If the compressor banging against the shell is releasing particles into the system, is there a filter or something other than a dryer that will catch the particles?

The compressor has rotolock connections, and I would like to avoid any cutting/sweating operation if I can reasonably-safely avoid it. Thanks for your help. - Doug


re: noise is coming from the compressor itself - a failing compressor motor indeed can become noisy;
I would not install a filter/dryer before replacing the compressor because

But if you wait for a failing compressor to burn up the result can be extra contamination in the refrigeration system.

For those reasons I'd ask for an experienced HVAC tech to look at the system, diagnose the problem, and then I'd decide if it were time and better to replace the unit now, or shut the system down and replace it before start of next season.


Thanks for the tips. All makes sense to me. I will consult with the a dealer that I know, before I do anything. I am inclined to just replace the compressor though, if I can find one at a reasonable cost. I found a new AM Std compressor on eBay that appears to be interchangeable with the one in my unit, but so far, I can't find cross reference data that I trust.

I will probably use it lightly until we get into cooler weather (6 weeks)and hope for the best, but prepared to replace the whole unit if crashes. I'll add a filter/dryer if I replace the compressor. Sorry if I didn't make it clear, I wasn't thinking of a two step operation.

I found a video on You Tube that shows disassembly of a compressor like mine. I don't think there is a broken suspension, but I would believe bearing noise. I don't understand how it can be so intermittent. Under certain conditions, something resonates and makes a real racket.
Thanks again for the help. - Doug

Regarding installation of a filter/dryer; the filters I have seen on HVAC parts sites are for installation on the suction side. When I installed an AC in my house in '88, I installed a filter/dryer in the liquid line, near the coil as recommended at the time.

It seems to me that it makes more sense to filter the liquid entering the TXV than to filter the gas entering the compressor. What am I missing? I purchased the compressor on eBay. It turns out that it was exactly the P/N specified in the parts list for the equipment. The gaspack contained what I assume was an earlier equivalent part. - Doug


Doug we see filter dryers installed at both sides of the compressor. It depends ... if a system is thought to be contaminated there are advantages of having a filter at both ends of the compressor motor, and it certainly doesn't hurt. - DF

Question: A/C System Buzzing noises for 2 seconds, bad start/run capacitor?

Outdoor unit makes a buzzing noise for about 2 seconds & stops. Fan never starts turning. I was standing by unit when this happened for the third time & manually started fan turning.
Fan started running on its on, but at a very low speed, nothing like it normally does. Any suggestions? Inside unit seems to work normally. - Hill

We seem to have traced this A/C buzzing noise to a bad start/run capacitor - Hill

Question: buzzing carrier AC unit goes BONK BUZZZ then runs "fine"

(Sept 3, 2015) Christopher said:
I live in an apartment complex and we have these really small CARRIER units from 1997. Our Apt complex told me " well if they're blowing cold were not gonna replace them. "

I just noticed tonight sitting here with no TV on nothing on around me when the outside unit kicks on I hear a loud and it kinda sounds harsh BUZZZZ like Electricity telling it to come on. Like right now it just went BONK BUZZZZ..... other than that its a very loud system but matience said oh thats because its a very old AC system.

We have been having alot of brown outs in this town and people have had stuff fried left and right I am wondering what that BONG BUZZZZZ noise is? Is it just the electricity hitting the fan motor and thats the fan coming on?



Sounds like a failing relay or control in the unit. I suspect repair or replacement is in the near future.

(Sept 3, 2015) Christopher said:

(13 hours ago) (mod) said:

Sounds like a failing relay or control in the unit. I suspect repair or replacement is in the near future.

Was that answer for me?

Was going to also say... After it goes BONK BUZZZZZ it runs fine..


Reply: yes.

Buzzing & Other Noises at Inverter Type AC Equipment

Question: Fujitsu Halcyon DC inverter (wall unit) that is making a loud sporadic buzzing noise

Joe Greaney said:

Hello, I have a Fujitsu Halcyon DC inverter (wall unit) that is making a loud sporadic buzzing noise. It sounds electrical in nature to me, like a transformer or maybe something short circuiting. There are four indoor units running off one outdoor unit. Only one of the indoor units is making this noise.

The system is new, only six months old, in a new house. The unit will make this noise even when it is powered off. I can't figure out a pattern to it, sometimes I'll hear it several times in an hour, sometimes I won't hear it for a couple of hours.

The installers were here and of course it didn't make the noise until after they left.

They were telling me there's basically nothing in the unit that could cause such a noise and suggested that it was that maybe just a grill rattling or the sound of refrigerant flowing through the pipes but I don't believe that, it's far too loud and besides it happens when the unit is powered off.

They are coming back Friday and I would love to tell them what to look for. Thanks very much!
Joe Greaney

Reply: things to look for when the AC is buzzing or vibrating

Buzzing when electrical power is off:

No insects, right? NO animals or animal nest, right?

I'm puzzled too. Even if we had an odd problem such as a late-releasing torqued motor component, refrigerant, relief valve, or fan motor, the sound ought to occur once, not repeatedly.

And refrigerant doesn't keep flowing through pipes when a unit is off. Certainly not several times an hour.

I would look for:

  1. Low refrigerant charge or a refrigerant metering device (TEV thermostatic expansion valve) problem causing freeze-up of the system; later thaw-out might be permitting some refrigerant flow.

    (The TEV will usually be inside at the cooling coil).
  2. Confirm that the noise occurs when power is OFF to the compressor motor.

    That ought to eliminate a failing compresor motor and also loose condenser unit parts such as a grille, motor or unit isolation feet, mounting, piping supports, failing contactor relays, bad fan motor, or other loose parts.

    That should also eliminate noises traced to improper voltage such as too-low voltage and a motor unable to start.
  3. Try covering the unit or cooling it off with fine spray from a garden hose, simulating rain on the unit.

    Does cooling the system change the noise? Inspect the electrical wiring to the unit including the condition of the outdoor service switch, circuit breaker, wiring for a bad buzzing connection - an unsafe electrical fire hazard.
  4. If your unit is a heat pump then check the solenoid coil switch that operates the reversing valve on your system - really ask your service tech to make these checks. For some heat pump systems the reversing valve solenoid may buzz for a time when the system stops operating - pressures are equalizing. It's normal.

Please see BUZZING sounds from A/C or heat pump and keep me posted.

Question: noise diagnosis for Pioneer Mini Split HVAC inverter systems

2016/11/17 Anonymous said [by private email]

Is there anywhere that describes the failure modes of inverter drives and their symptoms?

I have a Pioneer minisplit and it ran fine for several days after install but then the outside unit started to buzz.

At the same time the power use jumped from about 1,200 Watts to 1,600 W. This is 12,000 BTU unit and the heat produced is the same as before but the unit is using about 400 W more energy to produce it. Energy consumption tested with the heat setting as high as it would go for several hours. Outside temp about 50 inside about 65.

This unit is supposed to be HSPF of 10 and SEER of 21.

Comparing it to the energy used for my 2.5 ton standard heat pump, with a HSPF of 8.05, indicates the efficiency is much less than that 30 year old Trane heat pump. (I have permanently installed kWh meters on the heat pump and the minisplit.)

Reply: definition of VFD or variable-frequency-drive or inverter drive motors & their use in HVAC equipment

Thank you for an excellent question. We've written only a little about the use of variable speed motors (inverter drives, pulse motors, varible-frequency-drive motors) in air conditioners and heat pumps.

For other readers, an inverter drive air conditioner system uses a variable-speed electronically controlled motor or motors: synonyms: inverter drive, pulse motor, variable frequency drive or VFD.

VFD motors are also called adjustable-frequency drive, variable speed drive, AC drive, micro drive and adjustable speed drive motor systems. These motors use an electronic variable frequency controller to vary motor speed and in some designs to vary motor torque.

When diagnosing a buzzing noise in an A/C unit, including mini-splits and inverter motor systems I'd try to track down the noisy component by inspection and ear or perhaps a mechanic's stethoscope. For example a bad relay will buzz horribly but the repair is much less onerous than a motor that's failing.

However when you see the energy consumption rise, and thinking rather generally, after ruling out intermittent shorts and bad wiring, I do suspect a failing motor or fan bearing. I'm not expert on this but I speculate that any binding moving parts can increase the current draw in an electric motor.

The most extensive Pioneer service manual I've found is at where you can download your own copy at no fee.

I have read that noise complaints are not a surprise and in fact inverter type HVAC equipment manuals for some Carrier and other brand equipment has a "silence" feature: quoting:

8.4.16 Silence operation(optional) Press the “silence” button on remote controller to initiate SILENCE function.

When the Silence function is activated, the compressor running frequency will keep lower than F2 and the indoor unit will bring faint breeze, which will reduce the noise to the lowest level and create a quiet and comfortable room for you.

Much HVAC inverter repair material discusses troubleshooting VFDs and notes some common defects. Some of these can result in impalanced motor operation and vibration noise.

Sorry I can't offer a smarter guess. Please do keep me informed as what you learn will certainly help other readers.

More general information about inverter-duty motors

Watch out: The US DOE warns engineers and designers about VFDs:

Electronic adjustable speed drives, known as variable frequency drives (VFD), used to be marketed as “usable with any standard motor.” However, premature failures of motor insulation systems began to occur as fast-switching, pulse-width-modulated (PWM) VFDs were introduced.

The switching rates of modern power semiconductors can lead to voltage overshoots. These voltage spikes can rapidly damage a motor’s insulation system, resulting in premature motor failure. - U.S. DOE Motor Systems Tip Sheet #14 cited below

Continue Reading more Noise Descriptions at noise group 2 beginning with CLANKS CLUNKS from air conditioner or heat pump system


Continue reading at HVAC NOISE-2 CHATTER CLANK FAN GROAN - chattering, clanks, clunks, cyclical, fan, groans, growls or select a topic from closely-related articles below, or see our complete INDEX to RELATED ARTICLES below.



Suggested citation for this web page

NOISES, HVAC SOUND DESCRIPTIONS at - online encyclopedia of building & environmental inspection, testing, diagnosis, repair, & problem prevention advice.


Or use the SEARCH BOX found below to Ask a Question or Search InspectApedia


Frequently Asked Questions (FAQs)

Click to Show or Hide FAQs

Ask a Question or Search InspectApedia

Use the "Click to Show or Hide FAQs" link just above to see recently-posted questions, comments, replies, try the search box just below, or if you prefer, post a question or comment in the Comments box below and we will respond promptly.

Search the InspectApedia website

HTML Comment Box is loading comments...

Technical Reviewers & References

Click to Show or Hide Citations & References

Publisher's Google+ Page by Daniel Friedman