Intake elbow resonator also referred to as the noise damper on many Mercedes models using the OM642 engine. Includes new o-ring.

If you are looking at the engine this part is located in the front left, e.g. front of the right side bank of the engine attached to the head of bank 1. 

This part is prone to cracking, which will result in low boost and potentially limp mode, hissing noises on boost and you may see oil leaking around the part.