I was letting my friend take my rx8 for a test drive (after explaining some of the things an owner must and shouldn't do) and as I was sitting passenger side I noticed a rattling coming from behind me. On completely smooth roads nothing but here in Pennsylvania those are quite rare. After pulling over and moving to the back seat on the passenger side to investigate I could hear the rattling coming from the the back door or somewhere around there. Does anyone else have this rattling issue and if so is there anything i can do to fix it? It's a 2009 if that makes any difference.