I have found that such noise only at certain RPM is because of brinelling in the gears.

 

I have a noise in my dif which I am sure is normal. It's only when I purposely "float" the throttle and cause a no accel/decel condition. It's not speed specific like yours. Change the dif fluid, It will surely be black from break in. Change it after a long ride, it puts the metal particles in suspension in the oil so they come out. Don't let the dealer push you around. Go for a ride in another S if possible and see if the noise is the same.