Many Mercedes in the USA have a built in subwoofer in the rear deck. There is unfortunately a certain amount of rattle that occurs when the bass or volume is turned up.
I find this unpleasant, and go to great lengths to eliminate rattles. I've researched this a bit, and it seems some are caused by loose nuts in the trunk, some are from areas where the rear shade retracts. I found though a lot comes from the grill that goes over the subwoofer.
It's very easy to take out. Just a few tabs. It's a long piece of plastic with not much holding the tabs in place. I used dynamat on the back of all the tabs and along some of the longer areas without any musical output and along some of the sides.
It's a really nice DIY project that pays big dividends! I would also doubt they would even attempt to fix this at most MB dealerships.