Doesn't sound good at all

[NOMOFO]

Well I haven't been around here in I don't know how long cuz the truck has just been so darn trouble free while dealing with other crises in life. But.....
Today out of no where starts this horrible noise that is sort of random but can't help but think of rod knock or KDP and the like.
There are no puddles, no overheating, lots of oil pressure.
So I open up the hood and check the oil which looks perfect.
One obvious problem is that the serpentine belt ( which is only about 8,000 miles old ) has a big strip raggedly ripped off the back so I'm hoping it was just the belt chunk slapping around until it flew off.
The noise did diminish sort of instantly which kind of supported that theory. So I take the bad one off and check to see that all driven components are spinning freely which they are. A little resistance on the water pump but that seems minimal and likely normal. Then I get out the barring tool and rotate the engine manually and that seems normal with no unusual noises or resistance. I then put on an old belt that I had saved and then rotate the engine again with the alternator nut so my ear is closer to any of the most suspect culprits like inside the timing gear cover and such. I might have heard a little tiny something inside the cover but am thinking that was just teeth meshing while compression is peaking and I'm in between strokes with the wrench.
So then I'm brave or stupid enough to start it and it sounds fine at idle. I then go about 10 mph for 100 to 200 feet without noise then it starts again so I go back to the driveway. I turn it off and open the hood to look for disaster but nothing obvious. Start it again and idle sounds okay but when I rev it, it sort of comes and goes.
There was no loss of performance and still runs smooth as silk.
Well it's dark now and I'm not in the best state of mind so It's time to buzz you all and start fresh in the morning.
TIA for any thoughts,
Bob