Little brick bungalow in a midwest US city, built in 1954. New Heil
80% furnace and AC in 03-06. Standard forced air ductwork.
For a long time now, I hear a heat-related noise. About 20-40 seconds
after the burners fire, usually a big Clunk, like maybe someone striking
a duct with their fist. Smaller clunks sometimes when the ducts cool.
I've been chasing it for weeks. Lots of time standing on a chair with
my head nearly in the supply (grate off) for this room, that room. At first
I thought it was in the kitchen, so I tested there (no Clunk). Then in
the 'L. Then in the full basement where most ducts are exposed. Etc, etc.
Ducts are original except for a supply drop into the wall of a basement
bedroom built about 20 years ago.
Sometimes I fail to replicate the Clunk altogether. Other times I seem
to hear it somewhere else.
I know this sounds pretty fuzzy/indefinite. It's just standard galvanized
forced air ducts, but it sounds like it's in one place when it's in
another. Or it doesn't sound at all (except when I'm in bed).
Anybody conducted a successful gremlins-in-ducts investigation? :-) Not sure
what I'm looking for other than the source of a very elusive sound.