Bad "o" sound from f150

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.
F150 5.4l lariat 2004 bad sound - YouTube

 

Could be a number of things in the rearend.

But more than likely your carrier bearing in the rear end. I had a 05 that did that (not as bad as yours) I ran lucus synthetic gear oil and it quieted it down enough for me to live with.

Also sometime the ring and pinion scar from not being aligned properly. This will cause the wowowowowow noise your hearing as well when on the gas.

Need to get it up in the air on a lift and run it to locate the noise exactly first tho.

 

Just another thought, what kind of tires you running and when was the last time they were balanced?

 

Just stuff a banana peel in the rear end. Seriously though pull the cover and that problem should be evident.