Get under the truck, and listen while it's running..Also, have someone move it while you walk alongside, listening to it..
Note if the noise goes away when the clutch is depressed, this will help isolate weither it's the throwout bearing/pilot bearing, or something else..
While you're under there, check the exhaust for loose clamps, heat shields(as reccomended) and check the transmission fluid..(Note what the fluid looks like.Draining it into a clean clear plastic jug can help spot excessive metal contamination..)
Good luck-
Meplat-