Torque the bearing preload adjustment bolt on your stem cap just enough so that the knocking goes away.

Torque required is usually in the 2-3nm range.

If the steering feels rough or sluggish after tightening down the headset bearing preload bolt when turning your handlebars from side to side, you’ve over-tightened the headset.

Back it off an eighth or a quarter of a turn and check it again by turning the handlebars from side to side and pulling the front brake and rocking the handlebars front to back to feel for any knocking.