Topend "Knocking" sound

[beater69]

So I have a pretty noticeable knocking sound coming from my engine. It is only really noticeable above 2000 rpm and under load. I'll start with a little background, I'll try and keep it short .
I don't really know too much of the history of the engine, it came from the disc brake donor truck I got for cheap (82 c10). It ran better than my old one. So I cleaned it up, new gaskets, fresh coat of paint.
Now just recently after eliminating other noises, exhaust leaks, u-joints, etc... I got on the freeway and noticed this sound. It's not pinging, I've played with the timing, disconnected vacuum advance, etc...
I started to suspect maybe rod bearing, wrist pin, or piston slap but it doesn't change at all when I pull plug wires one at a time.
With a mechanics stethoscope I've laid under the truck and listened all over. Not much of any noise. Get up to the top and it comes mostly from the pass side valve cover. Pull the cover and I can isolate it to #8 intake rocker arm stud. This is after I already replaced all the lifters. No bent pushrods, I swapped the rocker arm with the one next to it. Adjusted and readjusted valves. Now i'm looking at pulling the head and seeing if there is something embedded in the top of the piston hitting the valve?!
Any suggestions?
See....I said I'd keep it short!