Rattle At Back Of Engine

Hi, as a follow up to my earlier post (Metallic Rattle) I have done some further investigating. It seems that it may be the heat shield behind the engine, the one over the header pipes. But it seems lower down, and its deffinately coming from between the engine and bulkhead.
If it was a manual transmission I would say it sounds like the thrust bearing on the clutch. If it was a heat shield wouldnt it rattle all the time the engine was running? not just when in drive or reverse.
Has anyone out there had a similar problem with their tribute or can anyone give me some idea what it my be?
This only really happens when engine in hot and only when in drive (or reverse) and holding it on brake. Put it in neutral and it stops.

Thanks