I have tested without dishes and opened the door during the knocking sound. It appears to be only when the lower spray arm is in motion. There was no banging when the upper was in motion. Could not see any physical contact while lower arm was moving. Could it be a water directional valve (since I notice it does have lower arm and upper arm separate control it must be a directional valve) that is opening and closing at the gate for directing the water to the lower arm? Any other theories?