Diagnose & repair hissing noises coming from an air conditioner or heat pump system:

This article describes the causes of hissing sounds heard at an air conditioner or heat pump compressor / condenser unit or at the air handler unit. We explain why you might hear hissing when the compressor motor stops or why you might hear other hissing sounds at the indoor air handler of an air conditioning or heat pump system.

For each hissing sound cause we suggest the probably repair action needed.

Air conditioner or heat pump noise diagnosis & cure: this air conditioning repair article series 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.

Hissing Sounds from the Heat Pump or A/C System, Diagnose, Repair

Common sources of hissing noises at the compressor/condenser indicating serious trouble

1. A failed or failing or sticking valve inside the compressor motor
2. Abnormally high pressures on the outlet side of the compressor motor activate an internal pressure-relief safety valve.
   
   Check that the compressor/condenser's cooling fan is running properly. If the condenser fan does not operate (to condense high pressure high temperature refrigerant gas back to a liquid) high pressures develop on the outlet side of the compressor. Check the fan start-run capacitor and the fan motor.
   
   See CAPACITORS for HARD STARTING MOTORS
   
   When an electric motor is having trouble starting you may hear humming sounds.
   Also see HUMMING SOUNDS, HVAC
3. Abnormal and very-high pressures in the compressor motor, possibly heard as a combined hiss and shriek.
   Watch out: if you have this condition turn off the unit and leave it off until it is examined and repaired by a professional, as high pressures risk a dangerous equipment explosion.
   
   See SCREAMING SOUNDS at HVAC
   
   and
   also HOWLING HVAC SOUNDS
4. A refrigerant leak. This ought to show up soon as an inadequate refrigerant charge as once a refrigerant leak has started it's going to continue to leak until the system is out of refrigerant.
   
   Hissing noises from a refrigerant leak can be heard outdoors or inside depending on where the leak itself is located: outside in the condensing coil, in the high pressure refrigerant piping, or at a pressure relief valve safety device, or inside in refrigerant piping, at the thermostatic expansion valve TEV, or at the cooling evaporating coil (in air conditioning mode).

Common sources of hissing sounds that are less likely to be dangerous

Other HVAC system hissing sounds may be harmless such as

5. Hissing at the thermostatic expansion valve, typically low in volume and short in duration, and traced to the TEV or cap tube
6. Hissing sounds from ductwork: look for an air leak in both supply and return registers. Even a small air leak can cause loud noises. This sound will occur when the fan in the the indoor air handler unit is operating, and it may change depending on the position of various duct system registers and dampers, blower condition, even air filter condition.

Of course there are other sources of hissing sounds in buildings, many of them, but not specific to the HVAC system.

Question: compressor clunk followed by loud hissing when the heat pump compressor/condenser turns off - air brake sound.

2016/07/17 Safie said:

I've been trying to find some sort of answer to what on earth my Lennox heat pump unit is doing when it shuts off.
It's hissing, so obviously related to pressure, but it only does it when it turns off.

When it turns off it goes "Clunk! PSHHHHHHHHHHHHHHHHH!" like someone is using air brakes for a second or two.

That's the only time it hisses...and it doesn't hiss like that every time. It still clunks when it turns off. It had refrigerant added about 6 months ago and no leaks were found.

I used the InspectApedia.com search box and I readHISSING SOUNDS, HVAC, there wasn't anything on it like what I'm experiencing.

Maybe I'm just wording my searches incorrectly on Google. Like I said I've never heard a heat pump make that extreme "air breaks" hissing sound after it stops. Just wanted to see if anyone else had had this experience.

This question was posted originally

at COMPRESSOR / CONDENSER DIAGNOSTICS

Reply: look for a failing compressor motor valve. A summary of sources of hissing sounds in or at HVAC systems

Safie, in the Hissing sounds article we list the causes of hissing at an HVAC unit.

In particular, as your hissing sound occurs when the outside compressor/condenser unit shuts off, that sounds as if there is a sticking valve in the compressor, but from the remoteness of off-site -chatting, I can only guess.

When the compressor is running it's producing high refrigerant gas pressure on the outlet side of the compressor motor. When the motor stops, if a valve seat is dirty or a valve in the compressor is damaged, that high pressure air may leak backwards through the valve into the low side of the system.

Your HVAC service tech will probably monitor system high and low side pressures to see if she can detect this; unfortunately for a hermetically-sealed compressor motor, if that's the problem I suspect the motor will need replacement.

Question: Hissing Compressor noises, ice on the suction line, hissing ssssss sounds from air conditioner

sir i check my out door unit i see the compressor is getting cold and after few minutes the amperes will going up start in 17 up to 20amp.,and it comes sound like noise, yes that was my compressor also, after few minutes suck line becomes ice and noise coming like ssssssssssssssssssssssssssssssssssssssssss. - Danny Carumba

Reply:

Watch out: Danny: Because SOME A/C compressor hissing noises can be due to dangerous internal over-pressure conditions, I'd ask a service technician to check the air conditioner promptly.

Please see our catalog of sources of hissing sounds in air conditioners and heat pumps, described at SCREAMING A/C compressor/condenser unit sounds. Such equipment should be shut off immediately.

Check for a bad fan start/run capacitor or motor at the condenser unit.

Reader follow-up:

Hissing sound and humming from my AC - It was the capacitor! Thanks! - Bingo

Question: Hissing in "surround sound" after installing a new high efficiency A/C and Heating System

I have a very high pitch hissing sound "surround sound" throughout the whole condo. We did install a new /better/efficient AC/heating system...some original duct and tubing is still in place. The problem is, we can't sleep with this noise! And cannot pinpoint where its's coming from: wall.

I need a professional with GOOD hearing to hear this. We will be putting some sound proofing on the garage back wall that abuts, unfortunately the condo (6 units attached) doghouse. We have had all items on the wall of the doghouse shut off and can STILL hear this high pitch hissing? Any suggestions to what we can do and WHO to contact?

Have had 2 electricians, and audiologist, etc. check this but their hearing is not like mine and I do not have tinnitis, because when out and about I hear fine and not the hissing. HELP please. I really don't want to move. - El

Reply: Track down sound source before installing soundproofing around an A/C or heat pump system

El, before installing soundproofing it makes sense to track down the source of the annoying noise - especially since the noise, if abnormal, could indicate failing or even unsafe equipment that needs repair.

We discussed hissing at the compressor condenser unit at SCREAMING A/C and warned about this problem in the article above. But not all hissing is due to extreme pressure. An A/C or heat pump compressor (of the reciprocating or piston and cylinder design) that continues to run with damaged internal refrigerant valves may make a hissing noise too. In this case the motor will continue to run, it may not even overheat, but it's wasting electricity and running more than it should.

An HVAC tech may diagnose bad A/C compressor valves by noticing that the high and low side pressures equalize very quickly when the motor stops.

Hi Dan ...thank you for responding to my question. I cannot track down the source - it's Surround sound - everywhere - not emanating from one spot unfortunately! I do live in a condo setting, had the Condo president shut off all electrical items in the doghouse: housing of the electrical, pumps, phone, etc. Shutdown could still hear the noise!

It seems no one can hear this ...except my family and none of us are in a business to locate the source.

We do have all underground lights, no poles. I have tried earplugs even, I don't have tinnitis, because when I am not inside the condo - no problem. Electricians and even PSNH don't seem to hear the very high pitch noise. If I could locate the source: one spot - I would gladly put a hole there.

I think until everyone else's' stuff starts breaking down so we can get a 5 for 1 deal this won't be fixed - and I know at this point the original builder is probably past the taking to court stage. Contractors who worked on these? To pass code IMO - barely. Any sources or websites you can suggest I'd appreciate. Thank you. EL

I would also like to mention that a Large PSNH box sits at the end of our driveway.

they opened it up and changes a plug of sorts (???) not sure what, but also, a lot of underground cables under our particular front yard:PSNH, cable, etc. Itwould cost a lot know it would cost a lot , but don't feel we should have to pay since it (source)unknown still, is the cause. We also, have to contact Dig Safe prior to planting..

.I would think that the cable's, etc. would be way under , at least 4 or so feet...does anyone know building codes on cable depth necessary to pass codes in NH? If so please post here and info. as to contacts:web page , phone numbers, etc.

It seems to have increased as far as sound over the past 4-5yrs. We have also had a PSNH person come out to our condo to check our meter and it says with limits of Section PUC 304.02 by NHP Utilities (NHPUC), Chapter PUC 300, rules and reg's for electric utilities - really. I feel like I've exhausted my resources and I am not in this field...any thoughts to help? - El.

Reply:

El, beginning at NOISE / SOUND DIAGNOSIS & CURE we organize some approaches to trying to track down odd sounds. To get anywhere with public officials or electricians they will of course need to either hear the sound themselves or confirm it with instruments.

Question: Hissing sound traced to a bad capacitor?

Hissing sound and humming from my AC - It was the capacitor! Thanks! - Bingo

Question: Hissing at compressor/condenser unit in hot weather, heavy load or after 5 minutes

I have a carrier 2000, don't know how many tons it is but here's the problem; I had a few hot days above 100 with abnormally high humidity. I turned on the AC in the early morning when it was relatively cool. It ran for 3 days straight with no issues. Then I noticed the air wasn't cool at all. Just warm and humid. I went outside to investigate.

The condenser was making an obnoxious humming and it sounded like the compressor had a blow of valve. It was hissing like it had a leak with a large amount of pressure behind it. I waited a couple days to turn it back on. It started fine the condenser sounded good and the fan started like it should and it was finally pumping out cold air. 5 minutes go by.

I hear the loud humming again and the hissing. So here is my question: Is my compressor shot or am I low on freon. Because it will over heat and shut down. Thanks for your help.

Reply: humming ususally means a motor is having trouble starting; hissing may be different.

Aaron: "The condenser was making an obnoxious humming and it sounded like the compressor had a blow of valve. It was hissing like it had a leak with a large amount of pressure behind it." suggests an overpressure at the compressor, perhaps a blocked condensing coil or dryer. Your system needs service, but the compressor is not necessarily shot - let us know what your tech says.

Follow-up:

Dan: Thanks for the tip. By doing some research (and I am no HVAC guy by any means) I found that the compressor would indeed over heat because of too much pressure. I looked at the a coil in the furnace itself and it looked fine. I don't however understand " the dryer" factor. I did wash the outside of the unit before i started it and the air filter is spotless.

Reply: causes of hissing due to HVACR compressor over-pressure conditions

Aaron: if there is an overpressure at the compressor, a cause is usually a blockage somewhere. I don't know all possible causes, but some examples are

- a clogged condensing coil at the compressor/condenser

- a dryer (canister installed on the system to remove moisture and debris) has become clogged (search our website for "refrigerant dryer" to read about these)
- a refrigerant metering device like a thermostatic expansion valve that is clogged or sticking

Other hissing sounds in HVAC systems may be traced to air duct leaks.

See LEAKY DUCT CONNECTIONS and also see VIBRATION DAMPENERS for a discussion of devices used to isolate noises produced by HVAC air handlers such as air conditioners and furnaces.

Reader Question: trouble tracking down a hissing noise at an Amana air handler w/ Goodman compressor/condenser - since original installation

3 months ago, I purchased an R-410A split system.
Goodman GSX130301B 2.5 Ton condenser

and an Amana AVPTC36C14 3.0 Ton Air Handler with Variable Speed with a TXV

Since day 1 it has had a hissing problem. It is loud enough that people ask me what it is. The hissing comes from inside the air handler. The hiss starts as soon as you turn the unit on.

[The hissing sound] goes up in pitch for about 8 seconds, stays steady for 2 seconds, then goes down in pitch for about 8 seconds, and then there is no hiss for 2 seconds. Then it starts again and continues the cycle until the unit is turned off.

The fan running by itself doesn't make the hiss.

I have had 3 AC guys look at this and they can't figure it out. They only have guesses and want to do expensive things to try to figure it out. I can't afford that.

What gets me is they can't explain the 20 second cycle. It is like clockwork.

Any ideas? Thanks, Mark - 6/21/2014

Reply:

Perhaps this is due to a sticking thermal expansion valve?

Reader follow-up: AC guy says "air in the system"

That is what I was thinking. Something wrong with the expansion valve.

The AC guys that have looked at it say it has air in the system because their gauges fluctuate from 279 and 304. But I would thing a bad TXV could cause that also???

I could understand air in the system making it act goofy, but to have something that is exactly 20 seconds every time sounds like a device of some sort messing up.

They want to vac the freon, put nitrogen in, then redo the freon for $966. Another AC guy said about $400. Either way, that is a lot of money for something that should be covered under warranty.

One of the sites I was on stated that you need to remove the TXV bulb before connecting the lines so you don't overheat it. They said if it is too hot to touch, you have already blown it. The AC guy didn't even use a quenching rag. But if it was blown, wouldn't it just stop working all together?

Reply:

Your reasoning about the effects of air in the system is correct. Air is also a contaminant. It not only changes the operating property of the refrigerant gas, it also may add moisture.

Failure tomprotectmthe TEV from overheating during installation also violates the installation instructions and is likely to damage the valve.

Really? In all events, even the company's explanation, even if it were correct - which the logic of which is a bit foggy - would indicate improper original installation.

Refrigerant can leak out of a system for a variety of reasons that cause leaks in different locationsn some of which could be an installation error, but ambient pressure air does not leak *into* a pressurized refrigerant piping system. (Air can be introduced into a system by improper procedures in hooking up test equipment. )

If you realize that air in the system would, be mixed in with a gas and thus change its properties would be distributed throughout the system the pressure variation is not explained.

Conversely a sticking TEV, often caused by contaminants (such as debris from oxidized materials due to overheating) might cycle in its regulating performance, as the valve can frost up, then defrost, or can open not enough, or too much, causing changes in the high and low side pressures that in turn cause the valve to open or close again in response.

Electrical or control board component,failures might also cause cyclic behaviors ofmthe equipment, but in that cas we'd probably see corresponding changes in motor operation that would be easy to observe.

When the compressor runs continuously but,system pressure is varying its the refrigerant metering system (or a bad reversing valve) that come immediately to mind.

If you let them clean and recharge the system first without changing out the TEV there is a small chance that such a procedure would remove a contaminant fouling,the TEV (though I doubt it).
If after that step is taken the problem remains that would support our explanation, not the one you were given about air.

Since in all events we are talking about installation errors that one would think were under warranty (if they will agree) you could then let them go ahead.

Watch out about insisting on a particular repair. If you do that the company will decry any responsibility for, the success of the job (and may adopt a sloppy attitude that subverts the insisted on approach even if it should have worked).

While sometimes it is cheaper and faster to replace or change several things at once than to complete a detailed and precise problem problem diagnosis, in general what you want as the consumer is for the repair contractor to own responsibility for a correct and economical repair, without shotgunning the repair approach by doing unnecessary work at your expense.

Reader follow-up: making a recording of air conditioner hissing noise

Thanks for all the info. I will try a few more calls and see if I can find an AC guy that is a little more logical.

I have the system down right now to do plenum repairs, but when I get it back up I will try to make a recording and do a little more detailed diagnosis. I don't have the tools to do much, but I can see if it changes with the outdoor temp or indoor temp.

I will let you know how it goes.

I forgot to ask this. There is only a thin piece of insulation wrapped around the bulb/suction line and the insulation is open on both ends. I thought that was a little strange, but I found some pictures of other goodmans that were the same way. Would it hurt to put more insulation to see if it changes the hiss?

Also, If I take the TXV bulb loose from the suction line (to help narrow in on the problem) while it is running, will that damage anything?

Reply:

I'd love to hear a recording and to add it to our diagnostic library. Moreso when we have a final diagnosis that makes sense of the sound.

This is a link to a youtube video of my AC problem. It is mostly just sound with little audio. I need to try a few variations to see what causes a change, but I thought this might trigger something in your mind from you past experience.

Are there any suggestions you have for making a better recording, like location of the mic, when it is cool outside or hot outside, cool inside or hot inside, windows open,.that sort of thing? Let me know if you can't access this video. I may need to make it public instead of private.

https://www.youtube.com/watch?v=i4iDxGiBfIM [this video is "private" and cannot be viewed at this link - Ed.]

I did notice that the compressor changes the sound that it makes just a little. Sounds like the load is changing. But it isn't as noticeable as the hiss inside. The recording makes the hiss sound worse than it is because I have the mic taped to the liquid line.

The liquid line is just a little warm and actually gets a little cool for a few seconds during the 23 second cycle.

Reply:

Have you tried a mechanic's stethoscope?

Reader follow-up:

Yes, but I don't have an electronic one. Had a hard time getting the mic to stay in position with the earpiece.
I took a chance and put some more insulation around the bulb and suction pipe. Now it sounds much better. Occasionally it makes a little hiss and sometimes the hiss will waver as if inconsistent freon is being pushed thru the txv. So, I can sleep on the couch now without the hiss waking me up, but the split is only 14-15 now (only 1 degree better than it was before the extra insulation).

I just did the work about an hour ago, so it may be a fluke. Maybe the hiss will come back. I will let you know.

I thought it was strange that this even worked since the other pictures I have seen of goodman systems have the same small amount of insulation around the bulb.

Since the second AC guy said the system was overfilled and vented some freon, I would assume that it was correctly charged at some point, yet it has always made the hiss. With that said, what would be your hypothesis now that the extra insulation has made the system quiet but not fixed the split?

Reply:

I'd figure the TEV is hissing, possibly not functioning perfectly. I'm not clear on why overcharging would cause that symptom. The TEV is metering liquid refrigerant on the high side of the system out into the cooling coil on the low side. It is designed to have liquid input and allow output of refrigerant that quickly changes to a gas - hence cooling the cooling coil.

Reader follow-up:

I agree with you. But I am not sure how to get Goodman to fix it.

I think the second AC guy was ignoring the hissing and focusing on the low split. From what I have seen, most AC people (with the exclusion of you and a few others) don't appear to truly understand how the txv works (not that I do either). So, they try to ignore the txv and blame something they understand.

Just curious, do you have an engineering degree in something? You appear to be much more intelligent than most AC guys.

The split was 13-14 when he got here and 13-14 when he left. And I still had the hiss. So he didn't do me any good.

I would think that, since the temp of the air going over the coils doesn't change very fast, the txv should stay fairly steady, unless it has a problem.

Is this roughly 20 second cycle what they call txv hunting?

Reply:

I have field exprience repairing HVACR systems, went to school on it, but am not an engineer and am not necessarily smarter than the other fellows you mention, but unlike some, I'm willing to read the instructions on the box rather than just use it to kneel upon. Also I've spent quite some time fielding and researching questions from InspectApedia readers.

The TEV is basically a thermostatically operated valve that opens to let refrigerant into the evaporator coil. Some of them are adjustable. Sometimes dirt or crud messes up the valve causing it to malfunction, ice up, or hiss.

We must keep in mind that guessing at a diagnostic when all I see is filtered through the pinhole of email means I'm just speculating.

PS: I wanted to add that your statement

"The liquid line is just a little warm and actually gets a little cool for a few seconds during the 23 second cycle."

may indicate a fluctuation in the TEV valve operation.

• Is the bulb making good thermal contact and is it secure?
• Might there have been dirt or debris in the system?
• Did the installer pull a vacuum before adding the original refrigerant?
• Did the installer include a refrigerant line filter/dryer?

Question: hissing popping sounds at the A/C system

(July 5, 2015) Charlotte Green said:
My ac unit just started making a hissing and popping/crackling noise when it would cycle off. The inside coils had a little frost on them. I turned the thermostat to off at heat/cool, but I turned on just the fan.

The air kept blowing so cold that I had to turn the fan off. It kept making the noise so I turned the breaker off. The outside unit kept running and I eventually had to go to the outside breaker to turn it off. The inside sizzled for a few more minutes, but now everything is quiet. But I'm getting hot. Any diagnosis?

Reply:

Charlotte

I would leave the system OFF completely as you are describing what might be an unsafe electrical component failure. The risk is fire or shock.

Question: weak hiss sound from the window A/C unit

(July 22, 2015) Zach said:
This morning my window AC unit suddenly and randomly began making an odd noise, it sounds a bit like a weak hiss, however, when I turn the unit on (with just the fan, not actual cooling) it still makes the noise as it spins up to speed.

It still dispenses air and cold air properly, however the noise is becoming very annoying, it sounds a mix between a hiss, a whistle, and a grinding noise. When the fan spins down when turning it off I can what sounds like the fan rubbing something, I've looked in the unit from the slits on the top, left, and right of the unit and I did not see any problem with the fan. I tried using some WD40 on the fan but no affect, it sounds like its coming more from the front of the unit that is in the house than outside where everything else is.

Reply:

Zach:

I am not sure if you are describing a refrigerant leak - in which case the hiss will have stopped by now and your system won't be cooling any more - or the sound of a failing motor or bearing.

If you don't see loose or rubbing parts I suspect one of those two problems.

Watch out: don't try running the unit with covers off lest you get a finger chopped off.

(July 22, 2015) Zach said:
I didn't remove the cover, i just used the extension of the WD40, and its still cooling perfectly fine, I've ran it about 3 hours total since it started this morning and the cooling capacity and air flow are not hindered in any way. The unit I have has 10 slits about a half inch to an inch wide where you can peer into on the three sides pointing up.

I don't have the money or skill to replace the fan, should I just run it until it stops working? I don't really have any other alternative.

Reply:

Zach

If you remain alert for smells, sparks, or fire hazards you might run the unit until it's dead. But I'm a bit uncertain just what got sprayed with WD40 (a wonderful product I agree - we just used it here in Mexico as an antidote for skin burning after handling hot poblano peppers). But I'd watch out: spraying some motor parts or winding with that lubricant can actually cause a failure.

(July 27, 2015) Anonymous said:
The fan shaft and there was a small opening on the base of the fan shaft, I sprayed both and saw no difference.

Reply:

Anon

If you are trying to lubricate an electric fan motor - and noting that not all motors can be lubricated - you'd need to find the lubrication port on one or both ends of the motor, typicallky a small diameter tube or metal cap that will direct the proper type of lubricant (electric motor oil) to the bearings.

Article series contents

• The following HVAC noise descriptions are a continuation from the article beginning at NOISES, HVAC SOUND DESCRIPTIONS
• HVAC NOISE-3 HISS HOWL HUFF HUM - hiss, howl, huff, hum, groan, woo wuu wuff or humming noises
  • HISSING SOUNDS, HVAC from A/C or heat pump system
  • HOWLING SOUNDS, HVAC from air conditioners, air handlers, blower fans, heat pumps, furnaces, duct systems & their motors
  • HUFFING SOUNDS, HVAC sounds from A/C or heat pump system, diagnosis
  • HUMMING SOUNDS, HVAC from A/C or Heat Pump system

...

Continue reading at HOWLING SOUNDS, HVAC or select a topic from the closely-related articles below, or see the complete ARTICLE INDEX.

Or see these

Recommended Articles

• NOISES, HVAC SYSTEM DIAGNOSIS - home
• BURNED-OUT COMPRESSOR
• BURNED-OUT COMPRESSOR FAQS
• CAPACITORS for HARD STARTING MOTORS
• CONTACTOR RELAY DIAGNOSIS & REPAIR
• CONTROL CIRCUIT BOARD, A/C
• COPELAND ELECTRICAL HANDBOOK [PDF]
• CRANKCASE HEATERS
• FAN, COMPRESSOR / CONDENSER UNIT
• FAN NOISES HVAC
• HARD STARTING COMPRESSOR MOTORS

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