i have a similar whining sound, but mine is coming from the a/c compressor. the noise is consistent whether the a/c is on or not. i think it is the pulley.
maybe yours is bad too.
the weird thing is that my a/c works perfectly. i want to change the pulley but can't find anyone who sells the pulley by itself.
I actually replaced my A/C pulley last year. I also had a noise coming from the A/C pulley and figured it's the bearing. Take the belt off and rotate pulley, see if it makes any noise.
MB actually sells the pulley with the electromagnetic clutch in a kit. I bought this and replaced mine without removing the compressor. There is a DIY pdf somewhere on here for the MLs...
Changing it on the E-class was a little tougher, as you have less room for the puller tool. I managed to get it done though.
The noise was prominent when you toggle the A/C on and off with the "EC" button. If this is the case, I suggest you change the pulley, you will be very pleased afterwards (I know I was).
BTW, the dealer I bought this kit from had to order it. They don't usually have it in stock. Apparently, it costs more to get the pulley replaced than the whole compressor at the dealer (WTF?). As far as I remember, it cost me ~$120 for the pulley (I do remember that the core charge was outrageous, even more than the part itself, they really want the old pulley back).