it might be a clearance issue like,
the rocker slot binding on the rocker stud momentarily
or the spring binding at full lift,
rocker under side contacting retainer,
retainer touching valve seal,
rocker lock nuts contacting inside of rocker,
have you checked piston to valve clearance!
pushrod rubbing on guide plate or slot, in heads, etc.
you should not hear metallic clicking if theres zero clearance issues,
once valves are properly adjusted