If it is a rattle, and not a squeak it could be one of a few things. You can take off the seat back and see if there is a stray bolt or screw hitting something. Also there are some wires under the seat that should be kept in place with wire ties. They can rattle. Are your tires out of balance?? This will make any car rattle.
1992 500E (very soon I hope
1981 280GE SWB
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