My left sliding door has had a knocking sound over every bump on and off for the last 2 years . Ford dealer tried to adjust the door with limited success. This has been difficult to diagnose given that Ford seemed to have designed lots of good engineering features to engage the doors . I suspected maybe the runners were rattling . Anyway in the I seemed to have resolved the issue with a can of white grease spray from Toolstation (product code 10203 price £3-19p) . Sprayed this into the upper and lower door tracks and door catches. Car is now silent .