i had a similar problem with one of my old 75's ?
it got REALLY annoying !
after many times taking the wheel off, checking everything, putting it back together, i couldn't find anything at all ?
i gave up, and booked it into a garage, preparing myself for a huge bill !
they phoned, saying they couldn't find anything wrong with the suspension, or steering ?
the knocking noise continued , but, knowing that nothing was seriously wrong, i turned up the radio to drown it out !
a couple of weeks later, i was working under the bonnet, checking levels etc when i found the problem !
one of the rubber mounting bushes for the airbox had broken off, and the airbox was bouncing back and forth in the engine bay !!
my 75 i have now has a k&n induction kit fitted, and the brace bar isn't strong enough to hold it steady, so it does exactly the same thing !
no guarantees, but worth a look ?