Mine too.

Is it new? or always there? sometimes could be springs if you recently replaced them. But if its new I'm betting its the head bearings need tightening. You won't be able to detect it except this knock over certain bumps. Simply feeling for slop in the front end just isn't sensitive enough. You need to remove the top yoke and lock nuts and see if the adjuster needs tightening down. I would start with that if I were you.

Put a zip tie on the slider down low and ride around as you typically do to see if you are bottoming under hard braking - doubt that is it.