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