250c

I have a whining/ electrical noise coming from the ac compressor on my 2012 c250. It does not vary with engine speed so does not seem to be a bearing or mechanical noise. It randomly occurs and the sound pulses on and off. Honestly sounds like the humming noise a pwm controlled motor will sometimes make(if you've ever heard that) which leads me to believe it is related to the electrical solenoid controlling the variable displacement of the compressor. I should also mention that pressing ac off button in car stops the noise. Have mentioned it to the dealer while I had my car in for other unrelated things and they just seem to brush it off. Best I've got is they hear the noise and recommend replacing the compressor. I'm not under warranty and the idea of spending probably at least a grand to replace an otherwise perfectly functioning compressor doesn't sit well with me. Has been making this noise for probably 3 years so the idea that compressor failure is imminent seems like bs to me. Temperature does seem to be a bit of a factor however even in the cold (30-40 degrees) it will sometimes make the noise for a bit when car first starts and then stop. Also it is not related to engine being cold as it can also make the noise with the engine fully warmed up during summer. As I said its really a mystery so if anyone has any ideas or if your own car makes a similar noise I'd love to hear. I know Mercedes uses the compressor model fitted to my car on many other models so doesn't necessarily have to be a w204.

Ksun_w205

This in an old thread but hope you can still see this. I have a w205 C63 S and I think i'm getting the same sound your getting. If you look at the threads I created you can see one where I ask about a AC sound. I get this like electric humming sound when I turn on my AC and its usually louder when its 85+ degrees outside. It still happens at lower temps but just not as loud. But sometimes the sound isn't even there when I turn on the AC in my apartment parking garage but driving 2.1 miles to a gas station and getting gas, turning the ac on again it will have the humming sound. But every time I get to my destination, when I go out to hear the sound its not there. Is this the sound your getting?

250c

I'm sure you probably have this resolved by now but the noise in your video is not quite what I was experiencing as at least in the video it sounds like a constant whine versus the pulsing type whine I was getting. Either way I eventually just decided to replace my ac compressor on my own and this got rid of the noise for me (found a really cheap price for an oem valeo compressor) . I don't believe that my old compressor was mechanically bad and still think it was just the control solenoid within the compressor that had started to whine slightly. Unfortunately those solenoids are not sold separately. Interestingly the newer replacement valeo compressor had a slightly different control solenoid diameter when viewed from the outside so its possible they upgraded the design later. As for your problem I will just say that based on the volume of the sound you're experiencing I've heard many mercedes models making this noise for example a 2018 s550 we had used to make this noise very slightly although it was very minor and of course inside an s class not audible. So its possible some ac compressors mercedes puts in make this noise normally. To me it seems that oftentimes with mercedes it can be luck of the draw and sometimes perfectly good components can make noises while others don't. If you are able to do work on your own you can often figure things out with experimentation. If not dealerships in my experience are part changers and can do very little in the way of diagnosis beyond what is already documented for them by the engineers in Germany so be prepared for a very large bill if you are outside of warranty. Good luck.