I have finally found the duct work that makes noise when the fan on the furnace shuts off. It is found just above the actual furnace and would be a pain to take out. The sheet metal buckles when fan is on and then retracts when fan shuts off and makes a bang noise. My question is, can i crease the metal somehow with out taking it out ??? or is there another way of keeping the metal from buckling without creasing the metal? any tricks of the trade