When I accelerate I get a "hollow whistling" noise from under the truck. There doesn't seem to be any rhyme or reason to it, by that I mean it doesn't follow the truck as it revs, but it seems pretty constant when I am cruising. It goes away when I let off the throttle ( I can still faintly hear it) but when you hit the throttle it comes back. Any ideas on whther it might be tranny or carrier bearing, or maybe something else?

 

I just had a u joint from the driveshaft go out and the end caps where making a pretty constant whistle.

 

I thought about that as well, I will replace them and see what happens. Thanks