I had a rattle from the rear end when i first got my base boxster. Turned out that the previous owner had bottomed-out the tail pipe on a driveway or such and the chrome exhaust tip was bent just enough so that it would rattle and buzz against the muffler. I just bent it back and the noise went away.
I would ditto the comment on the heat shield as another likely cause, but it may turn out to be something a little more interesting if the car is running with 160K on it.... Best of luck,
Ed
