It also happens to my car but not a Smart. Maybe this can help your problem.
This screw was so loose that the washer was rattling on it:

I also tightened these up a bit in the glove box:

Rear passenger door rattled so I applied some foamy padding all around, following the TSB (
http://www.alldata.com/tsb/Vol....html) as a guide, applied painters tape before removal, and then applied the foamy door seal material according to the painters tape, came out clean looking.
Now if I could only solve the rear seat in the locked position rattle
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