The knocking noise is caused by a hydraulic pressure back up on the return side of the power steering system, 172.s have a small diameter pipe on the return circuit from the rack to the resevior, it has been increased in size on the 182, so if a 182 pipe is fitted then the knocking noise goes. some late 172,s have the larger pipe fitted already, you can easily tell which pipe is fitted by looking at the part of it where it joins the resevoir, the modified pipe slims down in size to fit the resevoir, the older type of pipe is constant in size. Rob.