If it rattles with the motor running, push the motor to one side and then the other and see if the rattle goes away. Does it rattle on a hard right turn or a hard left? If so, you probably have bad mounts. If they are original Mercedes mounts and it is less than 12 months (unlimited mileage) from the date on your receipt, Mercedes will replace them including labor under their parts warranty. (The warranty information is printed on the back of your parts invoice.)

At 135k I replaced the original mounts on my 92 300d (they are the hydraulic ones) with aftermarket mounts. The motor rattled just like in your description and I figured that the noise was something else since I had new mounts. I changed the transmission mount and checked the exhaust mount and it still rattled. Then I contemplated changing the lifters, the idler pulley and all sorts of things until a mechanic leaned in the motor while it was running and caused the rattle to go away.
Year or so after that (at around 165k) the mounts collapsed so I bought original parts from Caliber. When I installed them all of the noises disappeared.

In your case, the original mounts cost the same as aftermarket mounts, are higher quality and have a better warranty.

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Tom Hughes
St. Louis
84 300SD
92 300D
86 300SDL