Gremlins in heat ducts


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.
Will
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Wilfred Xavier Pickles wrote:

Sounds like the expansion and contraction of the ducts as they heat and cool. Fairly common in hot air systems.
If it always did this, It may be poor design or installation. If it started after the install, it can be a bracket or support that loosened or sagged over time. The sound reverberates through the duct making it difficult to track the exact location. Since it happens after the burner fires, it is most likely near the furnace and the heat exchanger. Look for separation, loose joints, missing screws.
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Is there an isolation boot between the furnace and the ducts, something that isolates metallically the furnace? If there is, then you need to be very patient and keep listening. You could also deliberately put something between the ductwork and the floor joists / whatever supports the ducts to distort the ductwork very slightly and see what the effect is. It could be nothing, or something even bigger than what you have, but you have to isolate the source somehow and that is how I would do it. Patience is necessary!
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wrote:

I had the exact same condition with a recent furnace change. What was found was the cold air return duct immediately ahead of the furnace were "pulsing" in and out during the furnace cycle. You could watch the wide face of the duct move in and out at about an1/8". Went the furnace cycle would either start or stop the duct would "bang". The way it was corrected on the furnace was to ramp up and ramp down the blower speed at the start and stop of the cycle instead of an instaneous on or off. Another possible solution, which wasn't needed, would be to add a stiffiner on the duct where the pusling was occuring. Apparently, the draft on my new furnace was significantly higher that the old unit, which caused the pulsing of the duct.
Bob
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On Tue, 5 Jan 2010 14:43:23 -0800 (PST), "hr(bob) snipped-for-privacy@att.net"
wrote:

The furnace etc was installed 03-26-06 when that winter was mostly over. There -was- a 'clunk' last winter, but not previous to that.
On top the furnace is a sealed Heil evaporator unit. On top of that is like a plenum that they crafted with the 03-06 install. No isolation boot.
For reasons unknown to me, every time I go down there it won't do it. I checked mounting of the ducts (by metal straps) etc: all feels solid.
I will keep looking, listening.
Thanks, Will
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Wilfred Xavier Pickles wrote:

When you go down there do you close the door behind you? That could change the pressure in the ducts enough to temporarily stop the problem.
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No, no reason to close the door. You're right, it could have that effect.
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Open doors equalize pressure? Interesting idea.
--
Christopher A. Young
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You're looking for a heat duct that bends, and flexes with the temperature change. When you find it you can drill a couple holes, and screw some S-cleat onto the side of the duct, and help stiffen up the sheet metal.
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Christopher A. Young
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I must have found it, but ...
While trying to pin the sound down, I started removing grilles from the supply ducts, and the clunks eventually abated.
Grilles had rope caulk around perimeter that had totally hardened. I replaced the caulk and grilles, one at a time, and, Praise Lawd, the clunks did not come back.
Wish I could have nailed the precise cause, but I'll settle for getting to sleep without the clunks worrying the hull out of me. :-)
Thanks, Will
On Tue, 05 Jan 2010 15:51:54 -0600, Wilfred Xavier Pickles
wrote:

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