When you did the rebuild with the new timing chain etc. did you check the small end of the rod?
With high rpm use, the small end of the rod will oval out (causing the knocking sound). Normally, on newer engines with loose/worn timing chains, this is an indication the engine has spent lots of time at high rpms - so a worn rod would be my first guess.
Of course, like everyone else said, check to be sure nothing's loose in the head first.
Regarding the rocker play - the rockers will move side to side on their shafts, but you shouldn't be able to wiggle them up and down on the shaft - this would indicate worn shafts and/or rockers. And this could definitely be causing the knocking sound.