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For the past couple of weeks there has been a periodic rhythmic mechanical humming occurring in my home that will go on and off for hours at a time. We're in a detached single family residence built in the 70s. According to a cheap oscilloscope app it's about 80Hz and repeats a little faster than 1 hum a second in a sort of "WUm-WUm-WUm-WUm-WUm" fashion. I don't have a measure of it's loudness but I can hear it over my ~20dB computer fans. We can only hear it indoors, even if we're outside right next to the house. It is louder in open rooms and near certain walls.

Things from other posts here that I've tried to rule out:

  • Something mechanical outside -- At first we thought it was the wood chipper from a work crew taking down trees across the block from us. But they've been gone for a couple weeks now and it's still happening. Additionally, if I go outside and walk around the neighborhood to try and pinpoint the sound I stop hearing it the second I step outside my front door.
  • Neighbor's AC condensers -- I've checked a couple times and they've been off at the houses to either side of us during the noise. Same with our condenser.
  • Something electrical/hidden fan/pump/etc -- The noise continues even when I flip the main breaker.
  • Overpressure in water pipes -- We'd had water hammer issues in the past and recently tested the pressure on our main line which measured 80psi. Finally, that must be the issue! Nope, had a new PRV installed a couple of days ago and the noise is still happening at 55-60psi. If anything, now it's louder.

I'm at a loss trying to figure out where this is coming from. Something plumbing-related is still my top guess but we tried clearing our pipes this morning and we could still hear it even with all the water drained from our home. I'm hoping someone here has advice on further steps I can take to troubleshoot this or an idea of who to talk to to get it fixed.

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    "we tried clearing our pipes this morning" does this mean you shut off the water at the main valve and opened all the taps?
    – FreeMan
    May 17 at 15:58
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    Is there any construction going on in your area? Machinery, dewatering pumps, etc. can all make such noise.
    – jwh20
    May 17 at 16:01
  • @FreeMan yes that's what we did.
    – 5E4ME
    May 17 at 16:05
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    Do you have a condensate pump for your HVAC? Some modern high efficiency HVAC are "condensing" units, meaning they produce condensate from gas-powered heat. I once found mine running nonstop on an exceptionally cold day. The pump line had frozen, causing the motor to run constantly (killed power to the HVAC unit once full). Those lines can clog sometimes if algae starts to grow in the line.
    – Machavity
    May 17 at 17:42
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    Good call on the trains. I'll just add that "the vicinity" can be miles. One mile away is easily within range for what @JimmyJames describes. I've had the sound amplified by large pane windows. Never experienced a flue as resonator. Fascinating.
    – jay613
    May 18 at 13:55
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I don't know if this is the same issue but I'll mention it because "WUm-WUm-WUm-WUm-WUm" is exactly how I would describe it. When I moved into my first home, I would sometimes hear a low resonating sound like you said "WUm-WUm-WUm-WUm-WUm". It really scared me because I thought maybe it was my furnace or something but it would happen even when the furnace wasn't running. I finally just got used to it but I couldn't figure out where it was coming from. It sounded kind of like how I would imagine a UFO hovering over my house might sound (no UFOs were sighted.) I checked an 80Hz test frequency video just now and it was similar in tone.

A few years later, we had a gas fireplace put in and after a while I noticed that this sound was gone.

So here's my best guess as to what it was: there's a train yard a few miles away from where I live. When the diesels come through, they are going very slow/idling and they make a low "WUm-WUm-WUm-WUm-WUm" sound. Around a certain frequency, this sound was resonating in the chimney. When the fireplace was installed, two insulated liners were added (intake and outtake) which changed the resonant frequency of the chimney and eliminated the sound.

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  • That's really interesting. It makes sense too, the noise is louder in rooms near the chimney and the previous owner had it boarded up/full of newspaper. I'll investigate this when I get home.
    – 5E4ME
    May 17 at 19:47
  • @5E4ME Maybe try stuffing some fiberglass bat insulation in the bottom of the chimney. Please let me know if this works. I'm very curious.
    – JimmyJames
    May 17 at 19:50
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    @5E4ME fiberglass being non-flammable, which seems like a better idea than newspaper. Just don't forget to remove it if you want to have a fire.
    – JimmyJames
    May 17 at 20:03
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    Adding some insulation to the bottom of the chimney helped! The noise isn't completely gone but it's definitely quieter now. Do you know what material your new liners are made of? We currently have a terracotta lining but it's cracked so we can't use it until it gets replaced anyway. I can't wait to kill two birds with one stone here.
    – 5E4ME
    May 18 at 12:51
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    @5E4ME I no longer live at that house but I think the liner is pretty standard for an enclosed gas fireplace. I'm not sure it's the insulation per se but the tubes were installed from top to bottom of the chimney and terminated in the fireplace unit. I'm not sure exactly what about this stopped the sound and it was just dumb luck. Closing the top of the chimney might also help. There's a style of flue that closes at the top and has a chain that hangs down to the hearth. Sorry I don't have a more specific solution here.
    – JimmyJames
    May 18 at 13:34

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