I am hearing the same noise (to a much lesser degree) from my truck's power plant. Mine is not yet what you'd call a daily driver - it's still a work in progress - so I have not really tried to locate the source.
This is what I have noticed:
It sounds like a metal-to-metal interference issue.
It only cranks up when I apply a sudden heavy load to the motor.
It tracks along with engine speed while it is audible.
It immediately ceases when the load is dropped.
The first thing that comes to mind is a flexplate bolting problem - but the engine has basically no miles on it since rebuild. The T/C to F/P bolts were properly torqued and made up with blue Loctite. For those reasons, I am not inclined to go that way.
But you might want to pull the cover and recheck your bolting.
My next thought was some kind of interference in the fan/shroud area. But that doesn't seem right either. The shroud is plastic and would quickly wear away at the point of contact - I think.
Also, I seem to recall hearing this noise before on other similar trucks. To deal with it, I just stopped hooking the truck up so quickly. It went away and I soon forgot about it.
I would look around the engine compartment and try to find something that could come into contact with a rotating external engine component. Remember, when the motor is quickly and heavily loaded, the entire assembly reacts to internal torque and twists slightly along it's axial center line (this is true even if your mounts are good). Look for shiny spots, loose components, etc.
If I can find the source of my rattle, I'll report back. But don't hold your breath.
