Uhhh, I'm gonna disagree. :huh:
Take it to you local dealership or certified mechanic and have them cold start it & listen to the sound. I'd bet you just need a valve adjustment (30-60 min of labor).
The cam and rockers and valve tip wear down, and the adjustment bolts come loose, leaving the rocker loose when the valve is closed. So when the cam comes around again, the rocker flies through the air until *BAM* it slams into the valve tip. In regular operation, it should just rest lightly on top of the valve and the cam should smoothly push it open. Once the engine warms up, all the metal expands just enough to eliminate that extra play, so the noise stops. As time goes by, the ticking will get louder and the engine will have to be hotter before it quiets. Out of adjustment valves make a very characteristic clicking / knocking / tapping sound, and they hurt both your MPG and top-end power. When were they last adjusted?
It's conceiveable that the knocking is something worse, but get the valves checked first!
