Filling in panels with foam...

[Blue Fury 10+ year member]

I put it in my trunk lid, made it ugly as you can see in the pics from my damp install, but it helped A LOT with the rattle. Don't get me wrong it still rattles especially right by the bottom center of the rear windshield but it used to be horrible.

 

[WrenchGuy 10+ year member]

Look into Marine type flotation foams.

Had a boat buddy make the suggestion and its less messy than Great Stuff according to him. I might find a couple spots and use some myself.