What is ductwork, oil canning ?

In metal ductwork it is a sound caused by the expansion (usually shortly after the blower comes on) & contraction (usually shortly after the blower stops) of the ducts. Ever hear a sealed metal drum when heated by the sun? Thus the name.

If you look at an old oil can you will note that when you push the bottom in to get oil, when you release it will pop back. Ductwork will do this too, when you start the air handler. The ductwork will pop one way and pop back when the air flow shuts off. It can be stopped by finding the spot that is oil canning and applying a stiffener across the spot, usually a small piece of angle iron.