I just watched the video as well that sound is the cloth slapping the side of the unit as well as the side of the vehicle. I remember years ago Ryko tried to incorporate a mitter curtain on the Monarch 1 with the butterfly arm design.
Pressed cloth as opposed to woven cloth does not necessarily hold soils if the right detergent is used that provides good lubricity and is free rinsing. It is also important for the cloth to be rinsed constantly to flush any soils on them. Foam products do an excellent job for tri foams but in the wash process they tend to not make as much contact with the vehicle's surface as well as cloth. Done properly cloth cleans better in the wash process especially in mitters.