Mystery noise in flue (no, it's not a critter)

[DaveC426913]

TL;DR Summary

A noise has started in my chimney flue that I can't identify

Just on the last few days a noise has started in my chimney flue that is quite peculiar.

Please read the full description before posting theories. I've consider as many as I can think of.

• The noise is a beating. It is a whamming about 10 to 15 times a second. Sharp, rapid.
• Lasts about a second. Not sure if it is always exactly the same length. It may occasionally last 1.5 seconds.
• It is an active whamming; not just flapping. It's too fast and too sharp to be just passive flapping.
• It does not peter in or peter out, and it does not change rhythm - at all. i.e. artificial - not biological.
  .
  
• The sound each beat makes sounds to my ear like a small plastic door (like a flue) slamming open and closed very fast. Each beat makes a "thoomp" sound, but so fast it's just "TBTBTBTBTB!"
• The happened about eight times within 15 minutes yesterday, and then stopped, and then once today so far.
• The fireplace is not functional - purely decorative.
• I don't really have an option to open the chimney and look up the shaft.
• The noise emanates from near the ceiling on the inside. From the roof on the outside.
• I went out and looked at the chimney but there's nothing to see. I was able to hear the noise from the outside so I can tell it is coming from the chimney flue, i.e. not from within the house.
• My house is built in the last half century or so. This is my chimney. That corner of the house is an angled fireplace (fake).
• 
  
  Drat, you can't see it in Street View:
  
  

There are only three (four) things I can think of that a noise even close to this:
- an electromagnetic doorbell ringer / damped fire alarm bell ringer
- a woodpecker
- a clapper:

I am not suggesting that's what this is, simply it sounds like nothing else. It's too fast and perfect.

Things it is not:
It is not a critter.
It is not the wind. (there was no wind either day)

Things it might be related to:
The only thing that coincides with the sound, the location and the sudden onset on this new noise is that we just recently turned the forced air furnace from heat to cool. Wondered if @russ_watters had any opinion.

 

[berkeman]

- a woodpecker

That's what it was when it happened to me. Is there a chance you couldn't see the bird when you looked if it was on the other side of the chimney top? What is the chimney top/guard made of? Ours was aluminum, and the dummy didn't seem to care. Eventually I was able to chase it away with tennis balls...

 

[berkeman]

BTW, you could ask a few neighbors if they hear the same thing. Our nextdoor neighbor had the same issue with woodpeckers in the past.

 

[DaveC426913]

Chimney is brick, topped with ceramic flues.

This is pretty close, but two flues, not one:

It has no cap as far as I can see. Not sure how that works.

If it were a woodpecker, it would have to be inside. When I was outside looking up, the sound was def coming from inside, muffled, echo-y.

 

[berkeman]

Can you get up on the roof and look down the chimney with a strong flashlight? If you see woodpecker divots, that would explain it. You should probably cap that chimney anyway (unless it's sealed) to keep the rain out.

 

[RMW]

Does the noise cease to happen if you turn the cooling function off?

The 10 - 15 Hz "wham" could be a pump or an off-balance fan associated with the cooling system.

 

[Baluncore]

It could be a bat, flapping its wings, prior to receiving takeoff clearance.

 

[DaveC426913]

Does the noise cease to happen if you turn the cooling function off?

I have yet to associate it with the air system. I only suspect it because of the location (in the flue) and the seasonal timing.

The 10 - 15 Hz "wham" could be a pump or an off-balance fan associated with the cooling system.

It really, really does not sound like anything coming up to speed or anything coming down to off. It's perfectly regular in period, pitch and amplitude - from the first thunk to the last - like an electromagnetic doorbell chime (if you muted the actual chime).

I know pump sounds from my old house. They sound like a pump coming online and going offline. This does not.

 

[DaveC426913]

It could be a bat, flapping its wings, prior to receiving takeoff clearance.

It would have to be a big bat.

 

[russ_watters]

It could be a bat, flapping its wings, prior to receiving takeoff clearance.

It would have to be a big bat.

It's only the large ones that require ATC clearance.

 

[Tom.G]

See if you can find a willing plumber with a sewer inspection camera to investigate the chimney interior. You might even be able to rent an inspection camera somewhere.

Oh, and please keep us updated on any results , or lack thereof.

Cheers,
Tom

 

[desertshaman]

A little stream of droplets of water sometimes gets into an electrical cable in a confined space... ?

 

[Dullard]

Assumption:
Ornamental fireplaces don't have flues/chimneys. Whoever built it intended it to function.

Hypothesis:
There is a potentially large advantage in controlling the source of the combustion air for a fireplace. Depending on the mode of your HVAC (heat, vent, cool), the 'best' source varies. Your fireplace has an active damper controlling combustion air - it is slaved to your HVAC Mode. When the HVAC operates, the damper initializes. A bad position sensor (worn out spot on the potentiometer) on the damper will cause the sound that you describe - in terms of both regularity and duration.

Second-Hand Evidence:
I'd love to claim that I just thought of this scenario. Sadly, this is the sound that I hear every time that I start my truck. The mix damper in my AC system has this issue. I actually have the replacement part in hand, but can't bring myself to take the dash apart to fix an annoying (but brief) problem.

Or not.

 

[Klystron]

I have yet to associate it with the air system. I only suspect it because of the location (in the flue) and the seasonal timing.


It really, really does not sound like anything coming up to speed or anything coming down to off. It's perfectly regular in period, pitch and amplitude - from the first thunk to the last - like an electromagnetic doorbell chime (if you muted the actual chime).
View attachment 360181

I know pump sounds from my old house. They sound like a pump coming online and going offline. This does not.

Perhaps the ornamental flue has a fan, not a pump. The lowest spectrum look likes a small one-speed electric fan fitted in the flue.

 

[sandy stone]

My vote is for a woodpecker, assuming that they are present in your area. Around here, we have one that often pecks on our metal flue cover early in the morning, presumably to announce his presence to other woodpeckers and to claim his territory. The sound is pretty much as you describe it (only more metallic.)

 

[DaveC426913]

Your fireplace has an active damper controlling combustion air - it is slaved to your HVAC Mode. When the HVAC operates, the damper initializes. A bad position sensor (worn out spot on the potentiometer) on the damper will cause the sound that you describe - in terms of both regularity and duration.

If I had to bet on it, this, or something like it, is where I would place my money.

 

[DaveC426913]

Perhaps the ornamental flue has a fan, not a pump. The lowest spectrum look likes a small one-speed electric fan fitted in the flue.

You would expect a fan to spin up-to-speed, especially if it is whamming away on something.

 

[Averagesupernova]

If the noise is regular enough, and you can get by without electricity for a bit, turn off the power and see what happens. You may even be able to be standing with your hand on the main breaker/switch and turn off the power mid-rattle. See what happens. My guess is it's a woodpecker.

 

[DaveC426913]

If the noise is regular enough, and you can get by without electricity for a bit, turn off the power and see what happens. You may even be able to be standing with your hand on the main breaker/switch and turn off the power mid-rattle. See what happens. My guess is it's a woodpecker.

Alas, it is not regular enough. It happened about eight times in the space of an hour on day 1, once on day 2 and, so far, not at all today.

 

[berkeman]

Okay, this is a long shot, but do any of your neighbors have a Ring doorbell camera or other camera that has a view of your chimney? If they do, you could ask them to take a look at the times you've heard the noise to see if any birds/bats were flying in or out of your chimney.

Alternately, if you have a WiFi camera of some sort, you could reposition it to have a view of your chimney...

 

[hutchphd]

Is it associated with the wind?? Seems like 10Hz could be a resonance of the chimney stack. Is the chimney cap solid and robust? The damper?

 

[DaveC426913]

Is it associated with the wind??

No wind. First thing i checked.

The damper?

Damper is a possibility, except it's so regular and short and ... whammy. Like an electromagnetic doorbell contraption.

 

[Rive]

The sound each beat makes sounds to my ear like a small plastic door (like a flue) slamming open and closed very fast

I would check whether the chimney is equipped with a butterfly valve/flap to limit unwanted airflow - and that whether there is any other possible source for sudden air pressure change in the house than wind.

 

[DaveC426913]

I would check whether the chimney is equipped with a butterfly valve/flap to limit unwanted airflow - and that whether there is any other possible source for sudden air pressure change in the house than wind.

A flap is my assumption. That's what it sounds like.

It has not recurred. I can only assume it is closely tied to the household air system switching from heating to cooling at that time.

 

[hutchphd]

I can only assume it is closely tied to the household air system switching from heating to cooling at that time.

Or if the chimney temperature s slightly cooler than its surrounding air, there can be pressure to produce a backward flow. This might set something aflutter.....maybe the AC has some lightweight plastic air control flaps?

 

[DaveC426913]

Or if the chimney temperature s slightly cooler than its surrounding air, there can be pressure to produce a backward flow. This might set something aflutter.....maybe the AC has some lightweight plastic air control flaps?

Yeah, but this is a pretty deliberate whammywhammywham. Like it's being powered, and like it's in a loop.

 

[Rive]

A flap is my assumption. That's what it sounds like.

On second thought, the frequency is too high (10Hz, did I take that right?). Just as you said, like it was powered...
Any chance for a chimney-length resonance?

 

[DaveC426913]

Any chance for a chimney-length resonance?

 

[Rive]

Like an organ-pipe, I mean.

 

[DaveC426913]

Like an organ-pipe, I mean.

No, I get what resonance is in a pipe, but of what?

Here, have you ever seen one of these?

You hold it by the handle and shake it back and forth and it makes a sharp, fast whapwhapwhapwhap sound.

You cannot make it slow down and you cannot make it quieter. it's either whapwhaphwhap or nothing.

That's what this sounds like, but up in the flue (so, hollow and echoey)

 

[DaveC426913]

I confess I do hear a similar noise a little bit when there's rain or wind. It's not the same fast whamwhamwham - it sounds just like a flapper you'd hear weakly opening and closing with a pressure change in the house.

So my best theory is that there's a butterfly valve that can be gently disturbed by pressure changes, but it also has a motor attached or something that whamwhamwhams it about at certain times - presumably when the A/C turns on.

 

[hutchphd]

Remember a clarinet is just a tube with a flap valve (the reed) at one end. The resonance comes from the tube length of the air. My alternate model is more like an harmonica where the resonant frequency comes (mostly) from the reed springy-ness .
Maybe you can put in a keyboard and serenade the neighbors. Sort of a big basso profundo Alpenhorn. Golly I wish my house was musical.......when we annex our 51st state perhaps I can buy one.......(small sardonic joke)

/

 

[Rive]

No, I get what resonance is in a pipe, but of what?

My thought is, that the frequency is too high for a butterly valve or such to be flapped by just airflow. Something more forceful might be needed, like a tube-length resonance rattling that flap.

Remember a clarinet is just a tube with a flap valve (the reed) at one end.

 

[mathwonk]

at our house the northern flickers sound exactly like that.

 

[DaveC426913]

Heheh it's not a bird

So, another piece in the puzzle: it is making the same tone during rain. This time, rather than the mechanically perfect beat, the beat is raindrop-random.

So, raindrops are hitting what certianly must be a baffle.

I assume the baffle has some sort of open/close solenoid that's operated by the central air system, but it is perhaps poorly calibrated (badly positioned microswitch), so it "thrashes" for a few seconds before settling to its proper state.

 

[berkeman]

So, raindrops are hitting what certianly must be a baffle.

You don't have a cap/top on your chimney?

https://www.amazon.com/YITAHOME-Chimney-Cap-Stainless-Adjustable/dp/B0BX7SDHWG?tag=pfamazon01-20