1999 Ford F-350 clutch (area) noise

Recently bought this 7.3 deisel f350 truck with a 6-spd manual transmission. The cluch and trans appear
to function properly. When the truck is running at highway speed and it gets to a non-load point there is a grinding type of noise from the clutch area. It's not slipping or jerking and when I put the engine back under load it stops. Sometimes when starting off without much load (pulling off slowly) the same noise is heard in 1st gear and stops when I shift to 2nd. Everything is functioning normal, just this noise.
I thought that it could be the duel mass flywheel, but I was told that the 6 spd doesn't have a duel mass flywheel. I don't have a problem changing the clutch assembly, I just want to know if that's the problem. Thanks in advance.