I have this really loud and annoying "humming" like noise coming from the rear of my pickup and I don't know what it could be. I took it to the mechanic not to long ago and they said that my rear differential was doing something weird but I'm not sure if this could be it. Here is a video clip of it, at 70 mph the noise basically goes away but when I get to 65 and below it's very noticeable.

http://www.youtube.com/watch?v=VelnszPllyc

 

I would pull the rear Diff cover and look see if your gears are chewed up, then replace the fluid.

 

That differential is TOAST!

 

Yup mine wasn't that bad. Hell I couldn't even hear it. I took it to a drive line place and they took it for a drive and said its noisy we need to fix that. They were surprised I couldn't hear it. They do that everyday though. My rear was coming apart slowly.

 

sounds like my buddies truck did when the diff lube ran low and it toasted the pinion bearing

 

Mine sounded like the Millennium Falcon when it was about to go. Pretty awesome noise really.

 

The humming noise is probably pcfixer coming up behind you. Don't bend over or your rear will be TOAST

 

^ he knows from experience.