on the central locking problem: check the wiring where it goes from the door to the shell. if i remember right its a single brown wire that tells the other doors what to do and if this is earthing on the chassis or crossing onto another wire it will mess things up.

on the knocking noise: (sounds stupid i know but worth a try) is the centre nut on the shafts at the correct tension? its just that you say it only knocks once which makes me think this.

hope this helps.