Rapid thumping noise in decorative chimney flue after switching to AC

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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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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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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.
 
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.
 
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