When the valve train is constantly making noise, check for the following, as necessary:
Retighten any loose rocker arms.
Remove any dirt or chips in the valve lifters.
Excessive valve stem-to-guide clearance.
Remove restrictions from valve lifter oil holes.
Incorrect valve lifter may be installed in the engine.
Valve lock(s) may be missing.
Valve lifter check ball may be faulty.
Valve lifter leak down may be excessive.
Rocker arm nut may be reversed (installed up-side-down).
Camshaft lobes may be excessively worn.
Bent or worn pushrods.
Excessively worn bridged pivots or rocker arms.
Cocked or broken valve springs.
Worn valve lifter face(s).
Damaged lifter plunger or pushrod seat.