When I press the gas I hear a whining, like whistle from the rear of the truck, when I let go of the gas it stops, sometimes it gives a little rattle noise, any educated guesses on what this might be, maybe low gear oil? bearing? Thanks

 

while driving or when parked?

 

Driving, it's very quite while parked.

 

I had a similar problem and it turned out to be the outer bearing, the only way to know for sure is to get it on the air and listen with a stethoscope.

 

So I got 2 opinions on the noise, first guy said that my gear oil was low and dirty, recommended I flush it and put new oil, second guy said that my rear end is bad and I should have it rebuilt, so I'm gonna do the gear oil change first and go from there. Sucks to be me right now.