that's funny, I'm in the middle of fixing the SAME problem on my wife's Mercedes C280. It drove me nuts for weeks because the sound DID NOT vary with fan speeds. but instead, applying the gas changed the sound. Releasing the gas stopped it. I was suspecting the drive belts even though they're kinda new.

the only thing that made me keep coming back to the HVAC is that turning the system off stopped the noise.

Bottom line is that I've got the fan out (drove it around that way to confirm sound/no sound) and will clean it where the shaft is mounted at the bottom. When I applied a WD-40 soaked towel to that shaft bottom while it was turning, the sound stopped. But it came right back, so I'm optimistic that a thorough cleaning will do the trick!

Hope this helps you out some.