Glad to hear that someone else hears the rear end (transmission) whine. At 70mph is what i would call loud. In my case its not necessarily when power is on or off, just in-between. adding gas or letting off makes it go away. At slower speeds you can hear it too, its just a lower tone, almost a groan at 30.

I've worked on cars all my life, and it doesn't sound like rear end gears, it sounds like the sun gear in the auto transmission to me... but i could be wrong. either way... its just a noise. Drive train warrantee would cover it i imagine.