if you have the newer style steering column, than you can easily rebuild it to stop that play / knocking.
i had this problem too in my steering column, i than saw (on the E36 rack conversion guide in R3V or E30Tech) that there are two types of knuckle.

the later one can be rebuilt since it has bolts instead of rivets.
anyhow - once you take it apart you can see that bushing #4 is missing / way-out worn out and causes the free-play in the knuckle.
than you can simply by a new one from a dealer / make new one using some ocolon or fill the gap in with some silicon (this is what i did)
after this - no more free-play / knocking and steering feels great.
Ron.