Mercedes had an internal bulletin about this, but it is an advanced repair.
Most times, I see one of two things. Either the injectors are slowly leaking down or....
The bulletin. There is an adjustment on the rest position of the air flow plate. IF there is more than 1mm of play when the engine is turned off, from the top to when the plate just hits the plunger, it should be adjusted. This should only be done by a tech that knows how because done wrong, you will have to take off the whole fuel mixture housing to push the rest pin back up and start over or it will liquid lock the engine and blow up!
ASE CERTIFIED MASTER AUTO TECHNICIAN
MERCEDES SPECIALIST 11 YRS