You need a stethoscope.
Rag in palm..push on each pair of rockers over the pushrod side to check for clicky lifters in the future while its idling.
Just line a rag and or cardboard to the oil from dripping.
You should be able to pin it down with stethoscope however.
One thing that is hard to locate is a crack in the underside of a header at the flange..it eluded me for years till I took them off for new ones and did a close inspection.
