I have a 2001 Wagon, for a long time, like since new, there has been
some noise from the rear of the car when hitting bumps etc. I didn't
think much of it, thought maybe it was a mud flap making the noise.
Yesterday I was working in the back area and noticed that one of those
square cutouts in the side panel was pushed in. I've wondered what
they are for as they don't open that I can see. I pushed the panel in
a little and reached inside and pulled out a square plastic frame of
some sort. The panel door was pushed in and I couldn't get it out as
it's hinged along the bottom.

I checked the left side little door and managed to open it and that
one has the plastic frame attached behind. Does anyone know what the
panels are for, a place to hide things?  

Anyway, I think I found my rattle after many years.