Rattling noise underneath?
 

My 2004 Boxster S has a very annoying rattle somewhere underneath. It's very noticeable on a bumping gravel road while lifting my foot off the gas, almost sounds like gravel in my underbody shield, yet I find no rocks there. Could this be something major??

Is it coming from the rear? Shields on the cats

+1 get under the rear end with a length of wooden dowling and push the metal sheld off the exhaust or you may need a couple of jubilee clips to go around the shelds on the cats.
Guvs.

Do it quick too, i pushed mine to bend it slightly to stop the rattle and it worked. One side anyway. The next day it wasnt there, it had fallen off somewhere lol. Like most cars the shields are held on by nuts or screws, just need bigger washers on as the hole had enlarged and the screw heads pulled through, my lexus did the same