Transmission whining when coasting?

I thought the wheel bearings were going bad but now I'm afraid it's the transmission. At about 40mph+ I can hear a whining/humming noise from the front end. When I let off the gas it gets more noticeable and if I put the transmission into neutral it goes away. Swerving from side to side doesn't make a difference so I'm pretty sure it's not the wheel bearings. Has anyone here experienced this and what was your outcome?