Hi Guys, First time posting in a forum ever, so apologies if any of my forum etiquette is rusty! Basically I'm having a complete nightmare with my 2011 C250 CDI (automatic & 60,000 miles on the clock) and I cannot get to the bottom of the problem. As the title says, I'm experiencing a whirring/whistling noise at low revs (between 500 - 2200) regardless of whether the car is in neutral or in drive, it almost sounds like a mixture between a kettle boiling and an electric drill. The noise follows the RPM up and down as you accelerate and decelerate before dying off. I have seen other posts with similar issues but nothing exactly like this so I decided to create my own to get the best help possible. There is absolutely no other ill affects caused by this other than the massively irritating noise itself, the car is running perfectly, however I suspect this noise is an indicator that something is about to give so I am very keen to get a head of this and get it sorted. I have already had the car at 3 separate mechanics all of which have been equally useless at firstly hearing or identifying the noise and secondly once they've been spoon fed on where it is and what i think it may be they have came up with fixes which have not worked. Within the last 2 weeks this is what has been checked/done on the car: New Idler Pulley New Water Pump Oil & filter Change Transmission Fluid & filter change Hoses checked for leaks (none found) The power steering fluid is full so it's definitely not that, as steering has 0 difference on the noise anyway. The noise does seem more prominent or noticeable when the engine is warm as well if that helps anyone identify anything, and the whirring is not constant, again only happens when you rev in neutral or in gear between 500 - 2500 rpm. My gut tells me the alternator is on its way out but the fact that I'm not suffering from any false starts, miss fires and electric issues of any kind sort of makes me question this. If anyone has experienced this or has any suggestions or knowledge they could share I would be greatly, greatly appreciate it. I have only bought the car 1 month ago and this has really taken the shine off owning it. Thanks in advance guys.