The three causes of the 'Dok, Dok' noise of the chain climbing the sprocket found so far are: a spring link wider than the chain by more 'thou' than necessary, a really sloppy fit in other words. Too tight a chain and finally chain misalignment. Tightness needs to be checked all around the full length of the chain by rotating the wheel and motor and must be set at the tightest spot. 10/12mm up and down in the middle of the run is about there. Misalignment comes in: Vertical and Horizontal planes. The cause of a horizontal misalignment is the rear axle plate not fitting within the batten and pushing the motor mounting plate out at one end. Vertical misalignment is often caused by the top spacer being the incorrect size and tipping the MMP in or out. Use a square on the base to check distance piece size. A straight edge laid along the side of the sprockets are a useful guide. Please let me know what you find.
BW's Richard