Latest Discussions : HVAC


06:31AM | 07/09/04
Member Since: 07/08/04
2 lifetime posts

My central air started up with a new noise last night. I was lying in bed and I started to hear this very quiet, but uneven resonance. At first I thought it might be a singing return vent (been there before), but it wasn't. It was a low/medium pitch humming coming from the floor vents in the bedroom. The bedroom is setup quite near the actual fan unit. The walk-in closet has the entryway into the crawlspace, and about 3 feet from there is the big shaft that comes down from the fan unit. So, the floor vents in the bedroom are quite close to all the mechanics.

Basically, the noise is coming from that big initial shaft. Something is causing it to act like a resonance chamber with the air being blow in. I have no idea why. It wasn't doing this 2 days ago. This home was just completed about 2 months ago and it just started doing this now. It'll do it with either just the fan going, or the fan + air conditioner unit. I poked and prodded but the cause of the sound is somewhere I simply couldn't affect.

Overall, this sound is not noticeable under normal conditions. But under sleeping conditions, it's enough to prevent me from falling asleep (UGH). I don't mind white noise at all, but this hum is uneven and fades in and out and is at a pitch that really catches the ear. Annoyingly, the hum seems to focus exactly where the bed is in the room.

So - suggestions? The house is still well under warranty so I can contact the builder and have him take a look at it. But I'm a little worried they're going to think I'm crazy because you basically the sound is really only annoying when you're trying to get to sleep. Thanks!


06:59AM | 07/09/04
Member Since: 07/01/03
549 lifetime posts
This is outside my expertise, but lets speculate on why a circulation fan might make noise in a new home. When your house was being built, the drywallers and painters probably ran the air handler to lower humidity and improve ventilation. Could dust and overspray have bypassed filters and affected the fan bearing? Were the filters even in place? Sounds plausible. On a new unit, could the fan bearing be delivered with marginal or lacking lubrication? Like other equipment, it was put together on an assembly line, with a known and accepted defect rate. Why not? I have bought new power equipment that required return or repair due to manufacturing defects. Like you, my first question was, did I do something wrong, or is this normal?

The HVAC installer should be called to inspect the noisy unit. It may require a bearing set or fan assembly, or perhaps it can just be lubricated. Either way, its not unreasonable to ask, and doesn't make you look like a complainer. Get your deserved rest and let the installer have a look at the air handler to see if any aspect of the operation is abnormal. Even if he tells you its nothing, you have documented the problem, so if it gets worse in the future, you can point to when the problem started.


07:10AM | 07/09/04
Member Since: 07/08/04
2 lifetime posts
Hmm. These are possible I suppose, but unlikely. I opened up the front panel on the fan unit and let it run - the sound isn't originating from it. It's coming from 3-4 feet lower in the shaft itself.

One potential thought I had was that this was due to the house settling/shifting. The kitchen floor has been doing a bit of creaking recently as the house arranges itself. I almost wonder if just a millimeter or two of shift has repositioned the ducting in just such a way to cause this. That's a shot in the dark though.

The truly odd part about it all was that 2 nights ago - completely perfect. Ran the fan all night and got nothing but the rush of air . Last night, bang, right away the hum started.


05:33PM | 10/29/17
Did you ever figure out how to get rid of that humming noise? I seem to have a similar issue and I'd love to get some inspiration about how to fix this...


11:12AM | 10/30/17
I agree with those who suggest your blower fan is off kilter. I bought my home 6 years ago, and the 35-yr old furnace (rated 99.9% efficient) was louder than I thought it should be. A rather skillful local AC tech agreed and he fiddled around with the actual fan - moving and adjusting it where it sat - and each time after would start up the fan to test the results. I could see some adjustments made the fan wobble and create a noise and even a vibration which could be felt in the floor above. Eventually... he found the sweet spot and the unit has been peacefully quiet for about 2 years now.

It's all about rotational inertia. Think gyroscopes. If the fan isn't balanced *just* right, when it spins it will introduce wobbles and vibrations which can be from mild to scary. Plus I imagine this destroys the efficiency of your unit (i.e. you spend more than you normally would just for running the unit).


03:46PM | 11/02/19
I'm late to this party but I'll post in case it helps someone else.

I have a buzzing noise coming from the return duct, right next to my furnace. Since I just repaired the exhaust vent and the furnace hadn't been run in months I thought it might be a bearing going bad but the noise is definitely coming from the duct. I put my hand on it - lightly - and it stops. Since there are no moving parts in the duct it has to be doing the same thing as when you put blade of grass between your thumbs and blow. Or, the reed on a clarinet or saxophone.

A common cause is a dirty air filer and mine is certainly dirty.


04:23PM | 03/15/20
Often it's not the fan, but the vent cover that is humming through resonance. You can test this by pressing lightly with your finger against the center of the air vent cover, if it stops when you press then it's the cover.


09:09PM | 07/10/20
Cover 1/4 of the return duct . If the noise goes away and you have a cheap stamped steel return duct replace with aluminium of same size . I've had hand systems do this. Or install a higher merv filter . This most likely high velocity over the reyurn grill fins causing harmonics


10:46PM | 01/30/21
My 2 years old furnace started making a horrible humming that was frying my brain and ears. In the end it was a feeble vibration caused by the current of the transformer, which was sitting in the metal box. As feeble as it was, the ducts worked as speakers amplifying the humming. The HVAC guy left the transformer hanging and the humming was gone.


08:33AM | 09/08/21
Can anyone explain why it sounds like a bug zapper at my return vents? I didn't hear it when I first purchased the house 4 1/2 months ago (15 year old house) but now I hear buzzing. When I went to the area when the area where my system is located and the buzzing sound like it's in the duct or filter area. The filter is new.


10:26PM | 11/30/21
I know it's way too late but I'm just leaving a comment here so that someone else who has the exact problem can get some idea.
I had the same issue in my studio of an apartment building. The apt maintenance people including a building manager couldn't figure out why and where the noise was coming from. Then they tried out switching on and off the building duct on the roof of the apt. When they turned it off, the humming sound stopped. As they couldn't stop the whole apartment ventilating system, what they did instead in my unit was covering the vents in my kitchen and bathroom. They almost sealed the vents. I know it sounds stupid but that was the only way to stop the noise.. Thankfully I'm not hearing it again, but I really hope it never occurs again..

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