I have a F150 Fx2 2010. I'm noticing there is whistling sound coming from underneath or rear end of the truck. Every time i stay steady on the throttle but if I decelerate or accelerate it goes away for a minute till I stay at a steady speed. Does not do it over 60 or if cruse control is set. Could it be my rear differential? Like to pick some members thoughts on this.

 

It could be the rear end....but I would sure rule out tire issues first.

Have you check the lube level and make sure it is where is need to be?
do you have a limited slip rear end? Do you get any other noise like on lower speed, as in turning left or right?

 

All fluid. Levels Are good. And tires are new 12k on them. Not sure if its hub bearings . you know the sound that a power steering pump makes when its bad or low Its like that. No matter if I turn or not. If I'm steady on the gas that's when I hear it. If I let off or accelerate it stops until I go steady on the gas again. I have not noticed any slipping.

 

How many miles are on this?

Does this have a mid- driveshaft hanger bearing? or Maybe a U-joint that dried out and the sound is traveling down the drive shaft??

It boils down to you need to get this pin pointed down...with a chassis mic...or secure a go-pro pointed to suspected areas and make several passes and see which position returns the most noise.

In the mean, time crawl under there and start shaking things to see if you can get it to replicate...or set it on stands to get wheels off the ground and block the front wheels well so it won't move and try to spin the rear wheels to see if get it to do it again

 

i am going to make a guess at a bearing either at the tires or inside the rear diff. a bad ujoint i would think would have more problems like a clunk in the drive line under acceleration not so much at a steady speed.
but there is one other thing it could be you could have your tires road forced at a tire shop and see what the road force rating is anything above 12 to 15 means that they either need to be replaced or deal with it.
road force on tires can get worse over time and heck some tires brand new well if they have been sitting for awhile can have road force. try moving your tires around and see if the sound moves with them or goes away