Check what heatshields you can before going to the dealers, check the heatshields that protect the scuttle from the manifold, and have a look at the rest of the system as it runs down the car. Good luck finding the problem...
I've got a rattle under the passenger seat too, never worked out where it was coming from. Comes an goes, when its there its loud and annoying but other times its not there at all. Let us know if they manage to cure it.
This rattle is what I get under the passenger seat, backing off at about 4,300 revs. It hasn't caused any problems yet, but if anyone finds out what it is, it'd be great to know !