Valve Tap: Use a High milage engine oil (prefibly Castrol) along with Marvels mystery oil, or Seafoam.
Spark knock: Tune up, Use Platnum Plugs only! (Prefibly NGK or Bosch) Be Sure to change the Fuel Filter!
Spark Knock #2: Bad Gas, Low Octane fuel,
Solution: Fill tank with 93 Octane fuel, add 1 bottle of Outlaw Octane booster, Should clean it right up, If not, You'll have to clean the gas tank.
Spark Knock #3: Fuel Pump or pressure Regulator (plain and simple, replace it)
Ok, The reason I added Spark knock to this list, is because many times people confuse spark knock with valve tap, Those are some ideas. Unless I can hear it I can't really tell you which problem what you are having, but if you take it to a mechanic, and he tells you plain and simple it's valve tap, try the high milage motor oil with the additive, if that doesn't help clear it up, Then it's time to have the head rebuilt

Have a Nice day!
Curtesy of my employer, Mikron Auto Service