http://www.slk32.com/pages/fuelthump/fuelthump.html

Problem: Clunking, knocking, clicking behind the passenger seat.

There is a common issue with most SLK 320s and SLK 32 AMG cars.
This related to the fuel and emission control systems in these cars.
There is an emissions valve located behind the passenger seat that can make a clicking, thumping or knocking noise.
This noise is best heard when the car is at a complete stop and the top is closed.
This issue is normal but some earlier 320s can have this valve updated to reduce its occurance.
The service bulletin # is 103/P-B 47.30/15 Shut Off Valve
Also may go by this number : 47/3045


Problem: Rough idle.

If your idle is rough in the 320 it could be the engine ECU.
It can be replaced with an updated unit.
The Service Bulletin # is 121/P-BB0765/14 ECU Update
Also may go by this number: PB07-65/14