Two things:
1. Steering neck is loose on a hard braking stance is when you hear the knock. "Every now and then?" See, maybe a loose front end would knock on any brake mode. Loose is loose, right... ?Where do we go now?
2. Front suspension settings are too soft. You, "every now and then," bottom out and have this knock like mine, as I hit the transition from road to driveway. I have to stand up to add more shock rebound to my knees and the sound goes away.
Those are my two guess is, "every now and then" it may not be the two above and might be some inn else.
Tormenting the motorcycling community one post at a time