I have completely removed the panel behind the seats and this drastically reduced rattle and increased bass getting into the cabin...
However, Im still gettin a rattle on hard hits, and I cant figure out where its coming from. The only thing unmatted in my car is the area underneat where the panels were removed....Is this small area unmatted, enough to cause rattle if practically every other square inch in my car is matted? Anyone else have this problem?

 

Check the top of the gas tank. This was rattling for me for a while (when looking down, it will have a little wiring harness running into the top of the tank, that is held on with a metal bracket). This bracket was rattling, so I just applied some silicone to the underside of the bracket, and reinstalled it. Worked like a charm. Can you isolate where the noise is coming from (play some bass music, and try touching several different areas, to see if any pressure stops the rattling). This should give you a good idea of where the rattle is coming from, and how to fix it.

John