Grinding noise from the engine compartment
 

I have a question about my car - a W204 C180 CGI 2012 with 180 000 kilometers.
There's a distinct grinding noise coming from the engine compartment when I release the gas pedal and the car is at low RPM. For instance, when I'm driving calmly in 4th gear, lightly press the gas, and then release the pedal, as the RPM drops, I hear the grinding noise.

• The grinding noise is audible regardless of which gear I'm in, consistently.
• During acceleration itself, there's no noise.
• If the acceleration is more intense and reaches slightly higher RPMs, there's no grinding noise upon releasing the gas, and everything seems fine.
• The grinding noise is quite loud, perhaps a bit louder than the engine noise during acceleration.
• It doesn't happen every time I release the gas, but let's say about 5-6 times out of 10.

Overall, the car runs fine, accelerates well, starts normally, and I don't notice any other concerning symptoms.

Do you have any idea what could be causing this?

Thank you!

Can you reproduce the noise with the car parked while looking under the hood as someone revs the engine?

I would hook it up to a Mercedes specific OBD scanner and check for any codes.

RWD or 4matic?

I can reproduce it but it happens way less frequently also its a RWD, went to MB dealer they hooked it up it had no codes.

What does it sound like exactly, to give us an idea what it could possibly be?

With it in park i was trying to rule out any suspension or drive train issues.

Have you closely inspected your drive belt pullies and tensioner?

Im not too familiar with the cgi engine.

It sounds like something is rattling/grinding. I was wondering if it could be something related to the turbo. I will inspect the belts and pullies.

Alternator pulley clutch ?

Driveshaft or driveshaft support bearing? Not in the engine compartment, but perhaps worth checking.