I haven't experienced that, I've been in 25-30mph wind with 40ish gusts, enough to rock my truck suspension. That's before and after adding my soft insulation. I only have a 5' truck bed model though so it's pretty rigid. We occasionally get rattle from the lid buckles in the wind.
I wonder if it's between the skin and a support?
For insulating the metal bars I've used coroplast sheets, ABS type plastic sheets (above next to the slide), and this 1/4" plastic foam stuff. Not sure I have a favorite, the coroplast insulates really well on top of my wool insulation, but is clunky to cut curves into so I'm not super happy with my trim job. I may switch to a thin ABS/HDPE plastic sheets since you can work it like wood with jigsaws, etc and you can mount things directly to it. Some on here used the HVAC tape + carpet squares to eliminate the baked potato look and it looks nice.