With Active steering it invariably results in a complete new AS column being required. Mine was replaced twice! With non active it is most likely the lower steering link where the U/J's have dried out.
I asked them just to relube the column bushings but the reply was they would have to charge for that but if they changed the column it would be done under warranty!

 

Is the knocking sound loud or suttle and does the steering wheel twitches slighltly when the knocking sound comes on?