We recently bought a 25 year old house. It uses gas heat. When it got
cold and the heat came on I noticed a very loud cracking sound when
the heat came on and again when it turned off. This is from the
expansion and contracting of the galvanized heating ducts in the
basement. The ducts are on metal hangers so they are not touching the
floor joists.

I have never encountered this problem in any other house I have lived
in. How to best fix this? Someone suggested putting a strip of plastic
vapor barrier between the hanger and the duct to allow slippage.
Couldn't that be a fire hazard?

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