I have an 04 F150 Crew 5.4 auto trans Lariat. Exactley at 70 mph I get this loud whine from the transmission or rear end. Once I get over 74 or 76 mph it completely goes away. If I let off the accelerator it immediately goes away until reaching that 70 mph again. If I push down on the accelerator the sound also immediately goes away. I have no other problems with this transmission. I now have 46,000 miles on this truck.

 

I'd check the right rear tire for anything bent from hopping a curb somewhere along the line. Then Universal joints (especially if you trailer anything that loosened either the U-Joints or the differential itself). Last would be rear diff oil and seals (45K is when I had to change that in an old 4X4 Explorer but not sure on the F150).

Respectfully guessed.

 

I had that whine, at 40-45 mph. I had blown my left rear axle seal and was down a few pints of diff fluid. It was at 60k on my 04. The whine is no longer. It could be so many things, make sure you double check the simple stuff, like a feathered tire. Good luck.

 

Had this on my Exped.. Turned out to be the pinion gear s/u. Had to have the rear end replaced. Mine happened b/n 60-65 mph.