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How many miles on it? It could just have something worn out. You should be able to take the shifter off from inside the car and see if something is loose or worn.
My old 92 had a rattle in the shifter before I replaced the tranny and it was the retaining clips (??not sure if that the correct terms for it??) that held the shifter tight. Didn't affect the driving, just a little annoying.
There are so many different shifter designs and I'm not familiar with what Mercedes does but, I've always been able to solve this problem no matter what brand of car or age of car. My first step would be to remove the shifter to get a look at how it works and see if anything jumps out as being broken. Second step would be to visit your Mercedes parts counter or a local import repair shop. Chances are one or both are familiar with the problem and will know what part is required. If you go to a repair shop for advice, it would be a good idea to leave the guy a five or ten spot for his time.
It's probably a worn out spring washer, or grommet, or teflon bushing. Shifter parts are ususally pretty cheap. Dealer parts dept. should be able to print out an exploded view of the shifter with all of the part numbers for you.
ANYTHING like that will have movement...
but a rattle means something has changed...
I actually fixed the same issue on my Triumph tr-6 years ago with a rubber band
for a LONG time though, I just rode with my hand on the shifter...
I mean if one finger can stop the rattle, I figured a rubber band under the boot would hold it well enough to stop it too...
then after high school I decided to look at it and it was a grommet that had just loosened up and allowed the regular movement to shake a cotter key on a pin that held the shift lever in....
simple fix... once I removed the rubber band and boot and took it for a drive to see what it was...
however, YRMV...
could be gears tapping each other....