125-200cc Knocking noise steering

When I go over normal bumps suspension works fine, but if I go over speed bumps or standing up when the bike comes back down there's a destinctive ''clonk'' sound, somewhere where the handlebars join up, they're screwed on tight, fork seals seem fine, all bolts are screwed in, is there only one bearing in the steering that could have gone? Last mot guy said it had ''notchy'' steering, is it possible the bearing has been smashed up and is not working? Never taken forks or suspension apart before so not sure how it all fits together up top, where the arrow points in picture is best I can describe where the clonk comes from

Make sure the head stem Nut is tight (the one above the top triple clamp). Mine bike did this once and that was the culprit. It was clucking lightly on hard landing and g outs.

thankyou for that, I have now ordered new bearings and seals as I don;t think they've ever been done in ten years so are probably shot, also seems to be why the steering feels like the front tyre is flat, I will be making sure that final nut is done up too when I'm finished