On my truck (99 silverado 1500) after about an hour or so of driving a weird rubbing noise starts and gets really bad. It almost sounds like two pieces of metal touching but it's not a grinding it's more of a "whirling" noise. I'm thinking it might be a hub but it doesn't change pitch when I turn left or right.

It also comes and goes, it isn't steady.

I know its not the best description but any ideas?