Tach is steady even during the problem, so it MIGHT not be the engine.
The horrible sounds get much worse under acceleration and drops off quickly when he lets off of the accelerator, even while the RPS are still high. That would make me GUESS that it COULD be related to exhaust pressure.
My best speculation is that its exhaust related...possibly a cracked manifold near the fire-wall...possibly a baffle within the muffler broke loose...possibly the media inside the catalytic converter.
If you know a mechanically inclined buddy, have them come to you to check it out or get it to a shop ASAP. A relatively experienced gear-head will be able to tell fairly quickly if its a bad thing or if its a horrible thing...no matter what, it aint a good thing.
WATCH YOUR TEMP GAUGE. if its timing or a busted rod, etc, the temp gauge COULD give you an indication of pending doom.
"HOPE is not a strategy."
2015 (August) JGC Summit 4wd V-8, Black / Black,
Full skid plates, NO DVD (my preference)
20" Polished with Fortura tires (until the tires I really want hit the market in Feb...then Im switching)