Only when the heat is being used[not the a/c]. When the heat has been satisfied and the furnace stops running, after about 5 seconds you hear a boom from the duct, something along possibly expansion or contraction? Any ideas on what to do to correct.
Re: Duct noise
When Ductwork is installed they usually have whats called crossbracing or diagonal bends corner to corner. This stops the popping noise that your hearing, if they built there own plenums they may not have crossbraced them. At any rate if you get a piece of steel 1/4 X 1" x L and attach it diagonally to the part of your Duct thats popping it should cure the problem. First I would inspect your duct work and see how easy it is to push in and out, find a real pliable piece of Duct, turn the furnace on and wait till it satisfies