And the problems begin..
 

Hi guys, got a 2009 c300 4matic sport yesterday. I thought the transmission was blown, but the transmission is fine shifts perfect. To describe the sound I hear around 40mph is like the car is trying to shift with high revs, but the tranny isn't the issue. The sound seems to be coming from the right rear part of the car. Like a loud rev type sound even without the foot on the gas, and in neutral.

Edit, just re read the question

Sounds like a wheel bearing to me

Hole in exhaust pipe ?. Put a rag over the end & to create pressure to see whether the noise changes.

If the noise gets louder while turning the car one way, and quieter the other, it's def a wheel bearing.

+1 on the wheel bearing. To test just drive the car and slightly turn the wheel left to right. You should hear some kind of change in sound. NOW being its the rear its harder to tell, so keep your ear open to a change of tune. If you do hear a difference even if its slightly, it means wheel bearing.

confirmed wheel bearing guys, thanks any idea on the how much for it? average

the bearing itself is cheap. depends who does the work, labor can vary from a dealer or indy shop. either a shop press by removing the hub or press while in the car. depends on what tools they have. Probably around $500 per side. if youre to close to me, I can do it for free plus materials. Ive done so many of them. I have a 20 ton shop press.

I wish I was man, I'm up in NY

Hi guys just changed the right rear bearing the noise is still there. Took it to get the alignment checked because i noticed uneven wear. There is camber out of the left front -1.3 degrees and left rear -1.9 degrees. Should I change the left bearing too?

Where did you buy this car from? Since you just bought it, can you take it back to have them replace the wheel bearing at no charge to you?

Hi Pete, I bought it from a private seller unfortunately..

I would suggest you take it to a professional and although you will be paying shop prices, In the long run it will be cheaper than trying to guess what the issue is. I see this all the time. My customers come in after they spend hundreds of dollars with no results.

Also your camber is not the cause of the noise. They do sell camber kits to adjust camber but they really are no help at all because they really don't adjust much more than .3 deg. If you have a camber issue I would recommend you replace all the front end parts. Sagging bushings are usually the cause (ex: control arm bushings etc). These camber bolts are just for the front FYI. Rears do not exists.