So I have a 83 pickup 4x4 with a 1980 20r. With a 4 speed I was driving up a small grade on hwy down shifted from 4th to 3rd and herd a tapping noise so let out throttle it went away till I got back in it at about 2800 ish rpm. So I pulled over and listened while controlling throttle at carb noise didn't happen till higher rpm.now it make tapping sound when it get warm or while bringing up rpm but it's intermittent and out of frustration I tried to brake it driving it hard for a good 15 to 20 mins up and down old logging road but no luck didn't brake and still can't figure it out. This is what I have done compression test every thing looked good, lashed values, changed oil three times once super thick 2 bottles of Lucas oil stabilizer and the rest 20w oil didn't change the sound or the randomness of when the sound happens one other with 1 bottle Lucas half bottle atf and the rest 10w30 let run for about an hour total in 20 min increments and the last with just 10w30. I have dropped the pan rods look and feel fine no metal chunks shavings or dust in pan no loss in power. When standing out side the truck from the side it sounds like it is coming from the bellhousing but stand at front of truck sounds like the front of motor. Also have pulled plug wires one at a time didn't change sound. Anyone input would help

 

Do you have good oil pressure?
Exhaust leak?
Carbon deposits in cylinders?
Timing chain?
Hard to say, but if something is angry and wants out you will eventually find out.
It sounds like you have eliminated a lot of possibilities, try a mechanic's stethoscope.
I think pulling plug wires would have eliminated a bad wrist pin or piston slap, but I'm no expert in this area.

 

That's what I figured also I have good oil pressure just put different header on I have cleaned the hole block looking for a Crack