Glad to hear you may have sorted out the problem. These are sources or rattles in the back I have found:
Bad cat, like others have mentioned
Baffle loose in the muffler
Heat shield buzzing against the frame
If you have a chrome tip on your exhaust and have gone over a curb and kissed the curb, it can bend the chrome muffler tip so that it almost touches the metal under it, and it will buzz. I suspect this last one is just for 2.7L boxsters with the single muffler exhaust with the chrome tip.
Have fun,
Ed
