Mercedes Repair Questions? Ask a Mechanic for Answers ASAP
I'd first try removing the auxiliary drive belt and turning every pulley and ancillary ( power steering pump , alternator etc) by hand. All should turn easily (with the exception of the AC pump) with no grittyness or wobbly pulleys- If you do find a sticky pump / alternator then it should be replaced or repaired
If the engine is stone cold you can briefly run it with no belts to see if the noise changes or has gone away, this will help show if an ancillary is the issue
Examine carefully the condition of the bottom / crank pulley as these are composite construction with a rubber element and its possible this rubber is breaking up and allowing the two metal portions to move.
Tried running without the serpentine belt before the turbo was changed and the noise was there. Definitely coming from around turbo/front engine area. Tried listening around with an engineers stethoscope and can't pin it down. High shrill squeal (similar to a failed water pump) when throttle blipped from idle to approx. 1500 rpm. Visual examination of the crankshaft pulley shows nothing, with the bottom covers removed I can see the pulley with the engine running and there is no deflection when the engine is revved.
I've had the stethoscope on the front cover and cam covers while having someone rev the engine and can't hear anything, plus there is no rattles or unusual noises when the engine is idling. Today I drove the car without putting any load on the engine and the noise was not present at any road speed, it only occurs when the engine is asked to work or revved from idle. I've also dropped all the plastic under body panels and had a good listen.