My 2008 Mercedes C300 sport suddenly developed an engine noise and I'm trying to find out where is coming from. The noise does NOT go away even after driving all day. I replaced the belt tensioner, and idler pulleys but that did not help.


Any advice you can provide will be greatly appreciated.

Thank you.

 

Have a watch at this video, its most likely your air pump. Take the front cover off and listen to it running, surely you will feel/hear it coming from the air pump. These are good quality pumps and 99% of the time just need cleaning, no need to replace it.

 

I actually don't think it's the air pump since the noise goes up and down with the engine RPMs and doesn't go away after 10 or 20 seconds. It's most likely one of the belt driven components, probably the alternator or steering pump (I'm betting alternator). Pop that front engine cover off and find a long screwdriver. Place it on each component with your ear at the other end of the screwdriver. Any noises being made will be transmitted up the screwdriver and you'll be able to hear them. Be EXTREMELY careful to keep the screwdriver away from the running belts and electrical connections. You don't want it to get sucked up or thrown by the belt, or short something out Alternatively you can buy a mechanic's stethoscope for <$10 that will make the hunting a lot easier.

Probe the casing of the alternator, power steering pump, A/C compressor, each of the belts, etc. The whining should be immediately obvious once you pinpoint it.

 

That sounds like an alternator. If you can find a shop to take a look at it for you. Using a stethoscope will be easy to find.