bigtruck... since the noise appears after warm up I would have to think the valves are set too tight. metal expands when heated so clearances shrink. now, that may not be the problem... hard to guess without more info. I had a rebuild once with some milled heads and domed pistons and it would knock the tip off the plug once warmed up.
81 chevytruck, you might also look at your flexplate. they can also develop hairline cracks next to the crank bolts. also, make sure the flexplate bolts aren't too long. they can stick out the inside of the crank and nick the back of the block.