Valve Mechanism or Valve Train Noises

A light tapping at one-half engine speed, or any varying frequency, can indicate a valve train problem. These tapping noises increase with engine speed.
Before attempting to judge valve train noises, thoroughly warm up the engine. By doing this you will bring all components to a normal state of expansion. Also, run the engine at various speeds and listen for engine noise with the hood closed while sitting in the driver's seat.
After a major engine repair or long periods of non-use, "Lifter Clatter" at initial start-up is normal. This should go away after 10-15 minutes of operating the engine at 1500 RPM. Operating the engine in this manner will purge all air out of the lifters.

Caution: Do not remove the camshaft covers to diagnose valve train noises because exhaust manifold temperature will exceed the flash point of the oil causing a fire with possible personal injury.

It the valve mechanism is abnormally noisy, use a stethoscope to determine which valves or valve train parts are causing the noise.
The causes of valve mechanism noise include:
^ Broken or weak valve springs
^ Sticking or warped valves
^ Dirty, stuck or worn valve lifters
^ Damaged or improperly machined camshaft lobes
^ Insufficient or poor oil supply to the valve train (low oil pressure)
^ Excessive valve stem-to-guide clearance
^ Worn valve guides.