What is e most common cause to an engine knock on a 02 Harley davidson road king with 50000 miles bike still runs great no check engine lights are on and knock follows the rpm level

I'm sorry, but I don't know a lot about the Twin Cam engines but they are not that much different from the Evolution engines. One of the weak points in any Harley engine is the rollers on the ends of the tappets. The small needle bearing goes bad and it starts to knock. It will eventually lock up and ruin the camshaft that it's riding on. In your case there are two cams and it will ruin the one it's on. With 50K on the engine, that would be my guess as the I've seen the bottom ends of Evo engines still tight after 70K miles. I could also be one of the chain tensioners in the cam chest but usually people describe that noise as a whining type noise. If you still have the original pushrods in your engine, you will have to remove the rocker box covers to get the tappet blocks out.

Good luck
Steve