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

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Discussion Overview

The discussion revolves around a peculiar noise emanating from a chimney flue, described as a rapid beating sound occurring 10 to 15 times a second. Participants explore various hypotheses regarding the source of the noise, considering both biological and mechanical explanations, while noting the noise's timing and characteristics. The context includes potential connections to the operation of a forced air furnace and the structural features of the chimney.

Discussion Character

  • Exploratory
  • Debate/contested
  • Technical explanation

Main Points Raised

  • One participant describes the noise as a "thoomp" sound, suggesting it resembles a small plastic door slamming rapidly.
  • Another participant proposes that the noise could be caused by a woodpecker, sharing a personal experience with similar sounds.
  • Some participants suggest that the noise might be related to the operation of the cooling system, speculating it could be a pump or an off-balance fan.
  • There are suggestions to investigate the chimney using an inspection camera or to check for woodpecker activity.
  • A hypothesis is raised about the possibility of an active damper in the fireplace being linked to the HVAC system, potentially causing the noise due to a malfunction.
  • Some participants express skepticism about the woodpecker theory, noting the sound's characteristics and the location of the noise.
  • One participant mentions the possibility of a bat flapping its wings, although this is met with humor regarding the size of the bat required.
  • Another participant suggests that the ornamental flue might contain a fan rather than a pump, which could explain the noise.

Areas of Agreement / Disagreement

Participants express a range of competing views regarding the source of the noise, with no consensus reached. Some lean towards the woodpecker explanation, while others propose mechanical causes related to the HVAC system or the chimney structure.

Contextual Notes

Participants note limitations in their ability to investigate the chimney directly and the uncertainty surrounding the noise's relationship to the HVAC system. The discussion reflects various assumptions about the functionality of the ornamental fireplace and its components.

Who May Find This Useful

This discussion may be of interest to homeowners experiencing similar unexplained noises in their chimneys or those curious about potential interactions between HVAC systems and chimney structures.

  • #31
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.
 
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  • #32
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)

/
 
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  • #33
DaveC426913 said:
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.
hutchphd said:
Remember a clarinet is just a tube with a flap valve (the reed) at one end.
 
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  • #34
at our house the northern flickers sound exactly like that.
 
  • #35
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.
 
  • #36
DaveC426913 said:
So, raindrops are hitting what certianly must be a baffle.
You don't have a cap/top on your chimney?

1751751047251.webp

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

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