Nothing immediately comes to my mind, sorry, but as I am, sort of a "rattle freak" I do chase all of the noises in the car to get rid of them. I understand how irritating it can be. So some observations:
1. it is sometime difficult to determine the direction where the noise is coming from in the car. The source may be in a different place vs. where you think you can hear it.
2. Exhaust - will behave different hot and cold. It expands quite considerably when hot and it changes the tensions in its suspension. May need to determine if the sound is there cold or hot. If when hot only, go to the tail pipes and kick them a bit or put a glove on and shake them a bit...
3. Does it depend on side windows being up or down?
4. Have you checked if any washer gets lose in suspension when someone is sitting in the car? E.g. the two plates on top of the shock absorbers that have rubber between them? Are they fitted "edge up" so that they do not touch each other?
5. Have you checked the hand brake lines and main brake lines, again, when someone is sitting in the car?
6. Any lose parts of the chair? Any broken spring hanging under the cushion (had it once in W463)?
7. Are all the door chromes tight?
8. Are the door locks tight? No lose washers there?
9. No lose washers/springs/levers in the soft top hinge and soft top cover lock?
And so on and so forth. People here were finding lose washers on drive shaft making random noise... So unless someone can really direct you, it may be quite a lot of systematic checks...
My rattle/noise finds: lose washer in the rear soft top frame, lose screw holding the soft top cover moulding, exhaust - several times, lose nut on the rear hanger, lose harness bracket under dash, window cranks, etc.
