I hate to say it, but if you have a loud exhaust, there will be some things that tend to rattle.

I have a friend who put on a Greddy Evo, and we took off many things in hos interior, and got most of it to stop, but even still, It was annoying as shit.

the one we couldn't get to stop, and the only one that was very noticable, was the one coming from the back pannel, not sure what it was, cause we got tired of looking, but you may want to start there, also make sure all your tools, in the spare tire well are secure.

When and if you find out what it is, please, let me know.

Later,
Aj