I noticed it too. It sounds like tension rods flexing while driving. It is high up behind the drivers seat nearest the wall and slide out.
I think
ToledoCharli spoke of installing a brace between the wall of the slide-out and the seat platform. That places tension on the slide-out and requires removal and re-installation between stops. If we could adjust the tension of the rails by loosening the rails, applying the tension, then re-tightening the rails. Doing this might keep us from having to use the brace all the time.