lifter noise? help

I have a 68 dart gts, 340 motor mopar lifters ,stock rocker arms car starts easily runs strong no metal filings in pan and yet their is a noticable lifter tick or tappet noise on acceleration but reasonably quiet at idle.I even doubled header gaskets thinking it might be a header leak did not make a difference so I removed headers and trued up the flanges where they bolt to the motor still noisey.I know some of guys have been wrenching on mopars for a long time and any help or insight into my problem would be greatly appreciated ps 5200 mi. on motor. thanks in advance Paul.