I'm wondering what the problem is, it sounds in 40 mph when i have drive, if i let go of the gas or go to a higher/lower speed then it will disappear, I have had it in a gear less and same sound when hitting 40.
I belivie it's from the rear axle part but not 100% sure.
It's like a "ooo" sound, heres a video of it, i had a bit of trouble of holding the speed of the sound but you'll hear it.