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 the valve train noises, thoroughly warm up the engine. By doing this you will bring all engine components to a normal state of expansion. Also, run the engine at various speeds and listen for engine noises with the hood closed while sitting in the driver's seat. The causes of the valve train noise include the following conditions: