Hello guys!
This is my first post here so I hope it is in the right section.

My car is a C250 CDI 4Matic 7G-Tronic 2011 facelift sedan with 197.000 kms on the clock.
The engine is a 2.2 liter bi-turbo diesel.

I have a problem that has been driving me crazy for the past 2-3 months. When the engine is warm, I hear a rumbling noise inside the car, best heard from the driver's seat. The noise is coming from the engine bay. It is noticeable only when idling or at low speeds (under 40 km/h). When the outside weather is warm, the noise can be heard even better. With temperatures of under 20 degrees Celsius the sound is not there at all. It also increases as the RPM go up, but it is pretty hard to notice. If I open the bonnet to locate the sound, I can't hear it because of the engine's normal sound. Two mechanics couldn't hear the sound at all and I wouldn't want to spend a lot of money just for the diagnosis itself.
Could anyone help or point me in the right direction? Which engine parts could generate a noise inside the car and can't be heard properly under the bonnet? I am sure the sound wasn't there last summer and was definitely not there when the weather was colder.
The AC compressor is not the fault because the sound is there even when the AC is off. I also tried listening for other pulley noises but they all seem fine. Also the drive belt is new so that probably is not the issue. The noise would rather come from the back side of the engine(closer to the cabin) than from the front but I could be wrong.
Something related to this could be the fact that also after the engine is warm, when coming to a stop in D(Drive) and holding the brakes, the engine vibrates quite a bit. The vibration goes off completely when I put the car on P(Park) or N(Neutral). But this kind of vibration also comes when the engine is warm and the outside temperature is high.
Somehow my problem is dependant on the outside temperature.

Car drives fine apart from this and I wouldn't say the engine has other problems or is underperforming. Sound is impossible to hear at higher speeds, over 50 km/h and it's gone.
Could the engine mounts or transmission mount be bad? I really need some help because I'm tired of this noise and it's frustrating to be unable to locate it. :frown

 

I would probably start by having the mounts inspected. But it could be any number of things.

In a previous car I had a odd vibration pattern that wound up being some broken exhaust hangers. As simple as it was it felt and sounded like the cars was coming apart.