horrible grinding noise when starting but not when running

i have a 1979 k10 Chevy pickup. ive gotten 3 different starters because i thought that the horrible grinding noise was coming from the starter grinding on the flywheel. but the flywheel has absolutely no chips in it at all. It also dont grind when its running. so i had someone turn it over while i was looking at it. and the flywheel turns real slowly and the starter lines up perfectly with the flywheel so it cant be that. what else would it be??