Bruce from Chatham, Ontario had a peculiar noise problem with his floor. His heating ducts were made by simply nailing sheet metal to the floor joists, and every time he walked on the floor the sheet metal would ripple, sending rock-music-like sounds throughout the house.
He solved the problem by screwing plywood sheets over the metal and squeezing some soft foam rubber between the two. That kept the metal from bouncing up and down. His patch worked, but why did it make so much noise in the first place? Look at any large sheet metal ducts and you will see an X crimped into the sheet.
This pre-stresses the sheet metal and gives it a direction to bend with expansion and contraction -- preventing it from snapping up and down. Bruce's ducts were obviously not made by a sheet metal worker. Some of the skills of good tradespeople are hidden in such small details as a crease X-ed across the face of a piece of sheet metal.
Follow this link for more details on sheet metal duct noises.