250c

Hi all,
I've seen a few posts about this but all of them are dead and none seem to have found a solution. My 2012 C250 has a very loud whine from around 1250 rpm to about 3000 rpm. The sound is very similar to an electric power drill and the pitch gets higher with the rpm. There have been many theories in the posts about this problem but no solutions hopefully this thread can finally solve it. Using a mechanics stethoscope I listened to the alternator, power steering pump, ac compressor, and the two idler pulleys at the top of the engine. This was done with the throttle held at around 1200 rpm where the noise is first evident. I did not hear the sound from the alternator, or ac compressor. Holding the stethoscope on the power steering pump body i was able to hear a whine. The power steering fluid is at the correct level and the steering functions flawlessly and smoothly. I did find a video in which someone claimed that the idler pulleys were to blame for this and despite the pulleys still appearing to turn nicely when off the car, there replacement got rid of the whine. When i listened to the mounting bolt on both pulleys i did not hear the whine as clearly as from the power steering pump but despite this i plan on changing the pulleys and belt anyways. It seems this whining problem is very common on these c250 and it is surprising no one has yet found the cause. Does anyone have any idea what it might be?

alecmascot

don't waste time and effort doing that until you have run the engine with the belt removed and see if the noise is still there.

250c

I did consider taking off the belt and running it to test whether or not the sound was still there but since I had planned on changing the serpentine belt and idler pulleys as preventative maintenance anyway I figured I would save myself having to take the belt off twice. I will definitely run it briefly with the belt removed once the pulleys come in before I reinstall everything. Anyone else have any experience with this noise? From videos of the car I have found online I hear a lot of C250's making the same noise as mine but then there are others that do not make the whine telling me that it is clearly not normal. I really can't remember when it mine started making the noise but I am fairly certain it was not making the noise when I first got it because I know I wouldn't have bought the car if that had been the case. Either way I will update this thread when the parts come in as to whether or not it fixed the problem.

250c

Just to update the thread I replaced the serpentine belt along with the two idler pulleys at the top of the engine. Pulleys were well worn and ready for replacement. While I had the serpentine belt off I ran the engine and the whine is still present so I can rule out the entire accessory drive as the cause. This makes things more interesting anyone have any ideas? I considered an exhaust leak but figured that would be a higher pitched whistle and would almost certainly get louder or more pronounced at high rpm. This whine, however, is only present from around 1200 to 3000 rpm as I said. Possibly a vacuum leak of some kind. This is very strange.

tekfoc

Possibly transmission oil pump if your driving an auto transmission.
There slightly noisy.