I did get out the stethoscope and listen all over the block/head/injectors but the only time I heard a tick out of the ordinary was next to block drain plug.
Kind of leaning toward the injectors. When I ran the diesel purge through it, it didn't knock. But since it didn't really get up to temp I figured it was temperature related. And it sorta is. Here is what I just ran.
Gatorade bottle as my source of fuel with return and supply hose in it.

1/8 Diesel Clean - 7/8 Diesel = Reduced knock but still very noticeable.
Stopped it and topped off the bottle with Diesel Clean so about 30-40% diesel Clean
It ran without any knocking until the bottle got low. I then filled it 2 more times with about 30-40% Diesel Clean each time. And the knocking was practically gone for that entire 30-40 min of running. Temps @ the head were 190F/88C (may need to look at a lower fan switch). I could every now and then barely start to hear the knock, but I noticed if I grounded the fan switch and held it, the very slight knock would stop after about 20 seconds of running the fan. Could I have some sort of pre-detonation issue related to a injector? and lowering the temp helps cure it?

So the plan for tomorrow is to pull the injectors and have them rebuilt this week. I'll update next weekend. Keep the ideas rolling. Cause if that knock turns out to be injectors I'll be shocked. But I sure hope that's all it is.