I've had a couple Volvo's that made that noise and it was caused by the center support bearing on the drive shaft. The sound is kind of a cycling whine that changes octave with the speed of the car.
I'm not very familiar with Mercedes yet and I'm not even sure if they have a center bearing on the drive shaft but if they do I would strongly suspect that's the problem. A test I've used on Volvo's to tell if the bearing is the cause is to use a grease gun with a grease injection needle and shoot some grease into the bearing through the rubber seal on its side. The noise is usually caused by a dry bearing and the grease will temporarily stop the noise. This should not be done as a permanent fix though.