It sounds like when you have a crystal glass filled partially with water and get your finger wet and move it around the top of the glass...if anyone of you knows what that sounds like.

Its inside the cab.
I THINK it might be the exhaust, which dumps out right before the rear axle, and the rear differential harmonizing together because it makes the high pitched tone at different frequencies at different speeds and when I let off the gas it stops. It's suuuuper annoying.

Any ideas?