Don't know about the rattle - but in answer to #2 - PSE has nothing to do with suspension.

PSE duties:
Lock / Unlock doors & fuel flap (not related to the self-close feature of the doors and trunk lid)
Release rear headrests in cars without reclining rear seats
Provide additional vaccuum reserve for HVAC under conditions of low engine vaccuum, or when HVAC used in REST mode
Provide pressure for lumbar supports in front seats
In early 140's - retract/extend the parking masts on the rear fenders of the car adjacent to the trunk lid.
Extend/retract the trunk lid handle
Extend the trunk lid latching pawl when the trunk lid is opened.

If the car has self-leveling suspension (I think most of the S500's did), it could be a rattling strut, but I've read that those are really durable and last a long time.

Get someone to ride in the backseat and duplicate the problem if you can - maybe you can further isolate the location of the rattle.