340 lifter noise

I don't think he has adjustable rockers and instead of going on here or ebay and finding some he swap head gaskets to move the head closer to the block.

Gee if I did all that work i would had also put new in new lifters--if not a camshaft.

Sure its not an exhaust leak, headers for sure can make sounds that seems like metal is knocking?

Could be a bad/dirty lifter, they do get dirty and sticky, marvel mystery oil is good to mix with the engine oil to free the lifters up