Using a stethoscope, the knock is definitely coming from cylinder #1 or #2.
When the motor is cold, the knock is almost undetectable. After running for about 2 min, the knock gets louder.
If you rev it to 1500 rpm and then dump the throttle, it will almost die and catch itself around 500 rpm where the knock ceases momentarily for 1/4 of a second, then it comes back.
I checked the flex plate bolts and they are tight and the flexplate looks just fine.
I pulled injector #1 and #2 and bore-scoped cylinder #1. The piston looks "too" clean and there are what looks like the circular pattern of a valve impression on the top of the piston. I can actually see the part number on the piston and the area next to the circular impression is shiney like polished aluminum. There was also a little bit of fuel in the bowl on piston #1. Can a hung-open injector cause a knock? Plausible?
It got late before I could really take a good look at cylinder #2, however, just looking in the bowl, it looks to have 10x as much carbon buildup.
The next task is to fully bore-scope cylinder #2, and then pull injector #3 to compare cleanliness/carbon build-up between 1, 2, & 3.
One more funny quark, when you put the truck in gear, it's an automatic, it will pull the motor down and almost die, and then recover. When you bump it into neutral, it will surge up to about 1500 rpm. Bad-fuel injector related????