CLUNK in ductwork

Greetings, Little brick bungalow in the midwest US, built in 1954, poured-concrete foundation. Standard forced-air HVAC, galvanized duct-work.
New Heil furnace/AC in 03-2010. For years, in the winter, t-stat will call for heat, and about 20 seconds after the blower kicks in, I hear a CLUNK from the area of the kitchen vent, kinda like someone smacked the duct with a board.
Back in the '80's I cut a hole in the duct, added a vent for a basement bedroom near the kitchen duct. Doubt that is connected. Can hardly hear the CLUNK in the basement.
I'm guessing the ductwork for the kitchen vent is somehow stressed-out, pops as it warms up. It is inside a partition wall between the kit. and a bedroom. Old-style plaster on steel lath.
What commonly makes galvanized ductwork over-stressed? How can one unstress it?
Thx, P
"Law Without Equity Is No Law At All. It Is A Form Of Jungle Rule."
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Yup. Might need to make some room for the expansion. Could be messy.
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On 12/11/2010 3:59 PM, tommycottreau wrote:

More likely that duct run got slightly twisted or racked at some point. If sound predates new furnace, does it predate the work you did? One of the joints slipped just a hair, probably not even enough to leak, but enough to make noise as the panels expand and contract. I'd try physically wiggling the parts of the duct you can see, or maybe temporarily brace the duct with a stick, to see if anything changes. Fix could be as trivial as adding some strapping or a few screws, but if it is truly buried in wall, you may be SOL on a cheap fix.
Some of the ducts in this place thump at times, but it doesn't bother me. Tells me the beast is alive. (The trusses over the addition tell me when the sun is going down.)
--
aem sends...

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What I did was 20+ years ago. New furnace was 3-4 years ago.
Theres a fair amount of extraneous noise here. Maybe it just didn't bother me before.

I went down, held a hand on the duct while the furnace cycled. Didn't do anything. Outside the wall the duct feels rock solid. Slightly mysterious.

Its the same here. But if you had my CLUNK, it'd speak to you, say "Somethings Wrong!".
I'm still investigating.
Thanks, P
"Law Without Equity Is No Law At All. It Is A Form Of Jungle Rule."
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I chased this. The CLUNK was actually in the bedroom vent. When I removed the register, it went away for a while. It's come back but it's so infrequent it's a PITA to trace (twice a week or thereabouts).
Thanks to aemeijers for advice. It's probably in the wall: I'll find it someday.
P

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