My living room with fireplace backs to an outdoor HVAC unit. It's pretty loud inside when it's running. It seems louder inside than out. I think it's coupling off the natural gas meter and line right behind the fireplace, amplifying the sound.

Great stuff foam seal gas line penetration

Contact HVAC repairs to reduce noises
and if over 17 years old, discuss ETA for a new unit
+ explore rubber feet for unit via repairman to reduce noises

Masonry walls between noises & house can "throw" noises away and don't have to be that tall
or anything hard-surface similar

Interior wall (& fireplace wall) hangings of heavy textiles, e.g. Navajo blankets, etc.
Carpet floors if not heavy throw rugs


Noises can easily enter through tiny wall holes - like mice - then resonate in hollow spaces, be they vertical walls or better - fireplace towers. Once the vibrations invade, then other items - like sheetrock - pick up "the beat" (like a drum head), then the whole room becomes a drum.

First rule of noise abatement is to attempt to solve it as close to the source as possible.
Since you can't blanket a HVAC outside compressor ... back to considering a new one OR if new, fixing its problem (hopefully only twigs, debris)

Sazerac said:

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My living room with fireplace backs to an outdoor HVAC unit. It's pretty loud inside when it's running. It seems louder inside than out. I think it's coupling off the natural gas meter and line right behind the fireplace, amplifying the sound.

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Is the fireplace brick or stone inside only or inside and out? My guess is two possibilities. My guess is the AC is resonating with some natural frequency of the building materials. It's not intuitive, but thick, heavy materials like brick and stone that you'd think are very soundproof can transmit a lot of sound if it's at its resonate frequency which is in the upper midrange area. So the "whiney" sound from an AC could be activating that frequency.

If so, what to do? I guess diagnosing this would be a srart. My thoughts would be to run a test by placing thick, soft material (eg mattress, foam, heavy blankets) between the AC and the house to see what difference that makes. The reason I say that is because those materials will block the higher frequencies the stone might be resonating at.

If that works, then what? Maybe build a false wall and stuff with similar sound dampening material?

Can you move the AC unit?

How old is the unit? We had a loud AF unit that was 22 years old replaced in 2020. I could hear it kick on from across the house. New one is so quiet I can't even tell when it kicks on unless I'm outside on the backyard.

Unit is not very old.

As stated it is pretty normal sound when outside but there is some resonance happening with some gas pipe or something.

They make sound blankets that go around the compressor (inside the housing). Not sure what stress this could cause, discussion seems mixed. I looked into it before but never tried it.

Sorry I never answered yours.

The outside wall and chimney is all brick.
Inside the fireplace is the brick which has been faced with a stone product. Walls are drywall with I assume basic insulation.

On this wall it is only the gas line entering the brick. Nothing else is in the walls but a couple electric drops.

There isn't much else to seal.

A couple summers ago - when it's worse with hvac running all the time - I put foam around the gas pipes and it *seemed* to knock down the annoyance slightly.

I was wondering if it would take in wall spray foam to get a good solution.

Fyi that foam on the gas will cause the pipe to rust through from the outside in especially if you have sprinkler heads in that area. Don't ask me how I know this.


What brand is the condenser ? Goodman ?


Can you post a picture of where the freon lines enter the house ? Do they go through brick ? Also where they enter the attic area are they resting on the top wall plate or is there armaflex between the copper and the wood ?

Sazerac said:

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Sorry I never answered yours.

The outside wall and chimney is all brick.
Inside the fireplace is the brick which has been faced with a stone product. Walls are drywall with I assume basic insulation.

On this wall it is only the gas line entering the brick. Nothing else is in the walls but a couple electric drops.

There isn't much else to seal.

A couple summers ago - when it's worse with hvac running all the time - I put foam around the gas pipes and it *seemed* to knock down the annoyance slightly.

I was wondering if it would take in wall spray foam to get a good solution.

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With all that brick, and you saying it seems louder inside the house than out, I'm guessing the noise is hitting a resonant frequency of the brick. Motors spin in a few thousand HZ range, which is probably close to the resonant frequency of brick and stone walls.

Anecdotal evidence: I lived one summer in Corps Dorm 2 and brought my stereos with me. I figured with all the concrete walls, no sound would get out, but when I turned it up, the bass was shaking me like never before. I figured someone would complain about that bass noise, but a guy came from the floor above saying the tweeter was screaming right through his floor. Turns out the concrete's resonance was a much higher frequency than the typical wood wall in a house. That's why all the bass stayed in the room and the tweeter went right through.

So, my best advice is still to test temporarily putting some large, soft "thing" like several mattresses between the AC and the wall to see if that makes a difference. If it does, then I'm probably right about the high frequency resonance. If so, I'm not sure how you would treat that permanently. If you can get in between the walls of the chimney from the attic (I can in mine), then maybe stuff it with fiberglass (or something that won't catch fire, if it's too close to the fire box or flue. Maybe build a false wall on the outside between the wall and the AC and stuff that?