Burl, I know I've seen the layers of the cab roof stuff. I think I took off the gray c-shaped edge trim for some reason. I can't recall why I would have been in there, though. I did change the TV to a flat screen, but that should not have involved it. In any event, what I recall was a layer of some sound deading mat on top of the steel, and the flat luan shelf/ledge on top of that. The previous owner had shoved a hand towel in there, so they must have been fighting the same kind of problem. I think I removed it though! We are not having the problem.
Anyway, my suggestion would be to take off edge trim in the middle and see. I'll be inside the unit this weekend and see if anything else comes to mind.
OH, I think I recall someone, maybe Daniel the OP, saying he had a problem with all the TV and antenna wiring bouncing around making noise. Could that be it?