Sounds like gear whine in the differential to me. "Goose" the gas a little bit while you're driving. You should hear it while accelerating and not hear it while decelerating. Mine does it at about 75 mph.

It's a warranty item so you can probably get it replaced if you want. I personally am going to live with it for a while because I'm not sure any of the dealers in my area are competent enough to replace it.