Strange rattle from rear on 2003 Boxster S

[Southtxfarmer74]

Hey there everyone..I hope someone can give me some idea as to what is going on with my car.... I have a 03 S with 89000 on the clock. When I drive the car at speeds over 15 or 20 mph, I hear a rattle (like rocks in a tin can) coming from the rear passenger side. Its easy to hear with the top down and is driving me nuts trying to isolate it. I have checked all the heat shields around and over the cats and muffler, the clamps, mounts and bolts on the exhaust as well. Hit on the cats and muffler with a rubber mallet and cant hear anything rattling. The rattle does not seem to be from exhaust but does sound like a heat shield might be loose somewhere. I have looked at the aluminum heat shielding around the back of the car, but can not find anything loose. Took the engine cover off and found the piece of aluminum on there really loose. I installed high heat insulation in-between the heat shield and plastic engine cover shell which helped some but still hearing that rattle. What could it be? I re-worked the brakes over the whole car using OEM parts. Pads, rotors, spring clips etc...I don't think the brakes are rattling.... Shouldn't be anyhow. Is there something I am over looking? I have raised the car up and checked the suspension more than once trying to find a bad arm or bushing, but can't see anything wrong there either. Even checked the little tail pipe to make sure it was not hitting on something. The sound does not seem to be related to exhaust note but more to motion of the car itself. I would welcome any ideas as to what I might check next.