noisy return air duct

My furnace sits right below my dining room. Due to the design, part of my floor joist cavity in this area is part of the return air system and a couple of the joist bays have been covered with sheet metal to create the duct channel. The problem is whenever you walk in certain areas, you can hear the metal flexing and popping.

I am curious, could I replace that metal with something else like ductboard or drywall? Or would possibly adding some of that sticky duct insulation deaden the sound?

Thanks....

Wrapping the outside may help some. I wouldn't replace it with ductboard or drywall, you would likely decrease the size of the duct and that stuff doesn't clean real well.

Brian, I had the same problem but in my bathroom. It was flexing and popping. What I did was nail the sheet metal down tighter right around the top where it fits in the register, it hasn't made a noise in 3 years now.