The Stovebolt Site Forums The Shop Area Engine Shop Clunking sound from down under

Well, I've tried to ignore this sound but it's starting to really bug the U-NO-WHAT out of me. When the truck is cold this noise sounds like something is loose as if my horn was barely hanging onto the firewall or the oilbath cleaner is flopping around. This noise only appears when the truck is in motion, so if I'm at a stand still and rev up my engine when it's still somewhat cold, the sound does not appear. Most importantly the noise disappears after about 5 minutes of vehicle movement.....

So it seems like something else such as the clutch, tranny, or what else gets warmed up and the sound goes away. As a passenger, my son says it seems to be coming from (he points to his toes) that area ? Minutes later during driving, it goes away.

I've looked underneath to see if anything was obvious but no luck, especially since the sound goes away after warming up. So it must be internally ? But where ?

Again, I don't think it's the engine since I cannot hear the sound when reving it. Only during movement.

How the heck do I trouble shoot this ????