What does it do when you depress the clutch while moving when the truck's cold? Accelerate or back off? Apply the brakes? Lots of things to try. Does it make the noise in reverse? Does it change when you shift? Or is it directly speed related? What about when you turn the wheels? Or apply the brakes without backing off the throttle?
Could it be tires? Don't know what you have, bias ply will take a set when sitting then thump until they warm up. I'm throwing a lot of questions out because there are so many possibilities to eliminate.
