I hope someone can help with this issue, it's so frustrating. My C250 coupe suddenly makes a loud, constant spinning/whirring sound inside the cabin that has never happened before. It's a very bristly sound, gets louder upon acceleration, and has nothing to do with the AC/blower motor. I believe it's the turbo, but never heard it from inside the car before. Here's what led to this issue:

Took my car to the local shop for an oil change, as per usual, only this time they cracked the oil pan when replacing the drain plug. So begrudgingly, they paid to replace the oil pan and install it. I left my car there a week. When I get it back, no more oil leak but now I notice this fan noise, a spinning sound it never made before. Can even notice a difference in the startup sound, it's higher pitched, with a little whistle. Then I notice this sound is constant, spinning faster upon acceleration. Like a whirring, bristly paintbrush in a fan kind of noise. Higher pitch as you go faster, doesn't go away when the car warms up. Just a very noticeable sound that was never there before and seems to be coming more from the driver side. I took it back and they claim it's my alternator going bad. It's not, had that tested, nothing wrong with it and would not be able to hear that from inside the car.

Took it to the Mercedes dealer, 30 minutes away. Two techs drove it, said they heard the noise, and after a quick inspection determined it was the main air intake hose/pipe needed replacing and that would solve the problem. After that was done, noise is still there. So they took it back and checked all around, listened to various points of the engine with a stethoscope, and say everything sounds normal and found no abnormalities. Just that I must suddenly have become more sensitive to this normal sound of the engine. Even pulled up another identical C250 next to mine and said it makes the same sound and that's just normal. I found this insulting, as I explained how I've driven this car for 3 years and it has never made this sound before, it's always been very quiet inside the cabin, the only engine noise you can hear has always come from outside. Now all I can hear is this annoying whine, whirring sound coming through to inside the car. They offered to keep searching, checking and replacing various parts until they stumble on the right thing. They assured me the car was perfectly fine and safe to drive, the sound was nothing to worry about and if there was some kind of air leak or vacuum loss it would trip an error code from one of the sensors.

I believe them that the engine is operating normally, sounds perfectly normal under the hood or underneath the car. No loss in performance that I can detect. But this abnormal sound is constantly there now when it was never heard before. I assume it's the Turbo but something that was blocking or shielding this sound from entering the cabin is no longer in place or is loose. The guy at the shop who changed the oil mentioned all the labor involved in replacing the oil pan, including removing the steering column. That area seems to be where the sound is coming from, hard to tell. But when riding in the passenger seat the sound is definitely coming from the driver's side. What could possibly be missing or improperly installed as a result of replacing the oil pan that kept this sound insulated before and now it suddenly enters the cabin? I know my car and I'm not imagining this sound, it was running perfect before I stopped for an oil change and now I can't sleep at night wondering what went wrong. Don't know where to go next, hope someone has some advice. Thanks in advance!

 

that's a rough experience. the work on the oil pan and steering column is a wild card.

gets louder upon acceleration, Higher pitch as you go faster, Can even notice a difference in the startup sound

is that engine RPM or MPH? standing still or driving?

drive in low gear and is it the same?
if engine rpm, try to remove the serpentine belt and run it, see if it still happens or not, and to eliminate the belt-related things.

 

Either way. It seems more obvious when driving, but can hear it the same when in neutral and revving the engine. Mercedes dealer mentioned trying the serpentine belt removal thing if I bring it back. Hoping to find some local expert that will recognize that sound and immediately know what it is, will follow up when I learn anything. Thanks, I welcome any suggestions.