DravenX

For a little bit I've been noticing this higher pitched whining sound while driving my 2009 MB C300 Luxury. I just hit 84K miles on it this week. I thought it could be the transmission. It was recommended for service due to a leak so I did that this past weekend and its just worse. I'm not really sure what it could be, maybe the engine but since I'm not a mechanic I have no clue. I had a loud sound on cold start but that ended up being the secondary air pump. I was able to do that myself since it was right there in front. I tried to see if I can find the same thing but haven't been successful. Any ideas? Troubleshooting I should try? When I had the transmission service done I asked them about it and the mechanic said nothing was wrong however that seems to be a trend.

Adi-Benz

Sounds like transfer case.

I had that problem, cost like $4000 for a transmission swap. Try not to drive it as much as possible. Find a mechanic who can get it in the air, up to like 75mph, and listen for the noise.

What you do after that is up to you. It's not a diy job.

W204Motorsports

If the whine increases as speed increases, comes from the shiftier area and lastly does not go away when you put it in neutral while driving, unfortunately your transfer case bearings are due for replacement.

This issue is common enough now that the bearings are available without having to replace the whole transmission. They are about 300$ for the 2 bearings and races. If you can remove the transmission yourself and bring it to a mechanic, it will save you 1500$ but its not a DIY job like Adi-Benz has mentioned. This job can cost you anywhere from 1500-2500 $ depending what shop you take it too. I would recommend finding a local shop that specializes in Mercedes, that way they will know exactly what to do since they probably have done hundreds of them already.

Good luck.