a lot of people have that problem with the rattling in the gas tank....i do also. but the rattling isnt audible from the cabin, so it doesnt bother in much.

the rattling in the rear:

dynamat or something like it will help. but you should see what exactly is making the noise. if its the actual panel vibrating, then dynamat is the answer.

if the panel is hitting against something else, you can stick a sock in between it or something.