It sounds metallic like it's the ring & pinion chattering. Seems to do it more when it's hot out. If I let off the gas anywhere betw. 30-60 and then slowly accelerate the chatter starts. It's getting worse and actually started to make the truck decelerate last time. Thought the rear end was going to blow chunks. Any ideas?

 

It sounds like the problem our company has had with a few of our 99 models. It ended up being the pinion bearing that went prematurely. Get it checked out ASAP to keep it from a larger repair bill when it does let loose.