I searched the forums and have not found a thread with anything the resembles what i am hearing. A very odd hum from the rear (2005 f150 4.2) that occurs at all speeds. Low speed it occurs regardless of load on the drive line. At higher speeds it only happens when i have considerable load on the drive line. All bearings in the diff were replaced less than a year ago when the main pinion bearing went to **** and the diff gears were in pretty dam good condition. Wheel bearings are not getting warm to indicate a wheel bearing. And when i let off the throttle it hums for a few seconds and dissipates as the tension on the drive line goes away. I even looked at the carrier bearing assy and it seems to be in good working order. I live pretty dam far from any type of service so i was just curious before i start trying to trouble shoot this any further.

 

Could still be a wheel bearing. Jack it up and turn the wheels listening for a click.