What speed are you driving when the noise starts? You might be experiencing a rod knock, which is much more serious than a little valve noise. Also, slightly over-advanced ignition timing can cause detonation at highway speeds, which resembles valve noise somewhat. Take a check-ride with a well-qualified diagnostic tech.
Jerry

"It is better to be silent and be thought a fool than to speak and eliminate all doubt!" - Abraham Lincoln
Cringe and wail in fear, Eloi- - - - -we Morlocks are on the hunt!
There is nothing noble in being superior to your fellow man; true nobility is being superior to your former self. - Ernest Hemingway
Love your enemies and drive 'em nuts!