The wall separating 2 bedrooms on the second floor of the house contains a return air duct. Recently when someone walks near the wall, there is a banging noise coming from the wall. And several hours later, even though no one is upstairs, we hear banging noises again. Do you think the duct is loose inside the wall? Or is there some other problem with the wall.

Thanks

I think that when someone walks on the floor, the horizontal framing members are flexing down slightly. The duct work in the wall is running vertical from the basement, through the first floor into the second floor, and go through the second floor framing, and must be attached somehow to those members.

So that when the second floor is pressed down, it causes the duct work to buckle, and causes the sound that you hear. Keep in mind the floor framing, and the duct work is probably only moving a fraction of an inch, but the metal cannot move that much without buckling.

And the flooring has I am sure settled over the years a bit, taking up what little room there was between the duct work and the wood frame it is attached to.

If the noise is annoying, then you will need to at least remove one side of the drywall to detach the duct work from the framing, maybe both sides. Stopping the noise may be a much larger job than you are ready or willing to handle.

If this is an old home, for that matter, even a new one, check for excessive settling of the floor framing and the columns in the basement that are supporting the framing, may be an indication of a more serious problem.