Rear brakes are discs!

But Im thinking now if it could be the anti roll bar links on the side it is knocking (front drivers side) I had changed the other side and front bushes for a light knocking that I had going over small bumps in the road last year. wondering now if it could be that knocking, being the weaker link now and a slight movement in reverse could be enough for the light sounded knock to be heard...and only seems to happen when wheels are straight...cheers any more thoughts!