Mercedes rear roaring noise

I have a 2013 Meecedes C300 AWD. Recently I've been hearing a roaring from the rear of the car when im at slow speeds turning either direction. Once i straighten out the noise stops completely. The shop suggested that i replace the front, rear differential fluids as well as the transfer case. Which we did 350$ later and its still making the same noise. Any advise on what it might be?