Elenor,
I remembered reading this thread and I'm at my printer right now .It just dawned on me that there are three plastic 'caps' covers, that are in the area of the bottom plates. they should not be causing a problem, and I would have thought you have checked them already but...
One is gray that is the access cover to change out the cutter assembly, which is on the right underneath the top edge plate. The other 2 are black, one on each inside side of the cart drawers covers. they aree quite loose (to be able to be removed easily to attach the Spectro assembly.
Could it be any of these that are rattling?
One more thing that could be rattling if you have the in the position against the stand, is the two plastic guides that hang down from the bottom plate that guide the paper into the basket???
David