[neallewis]

The slipper assembly bearings move within the central mounts. The noise is the slipper unit moving as the motor spools up and down.
You need to remove the top plate, place two small squares of double sided tape on each side where the bearings meet it. Screw the plate back down, which compresses the tape against the bearing outer race.
This seats the slipper unit correctly taking up any play, and removing the source of your noise. You may have to replace the spur gear for a silent transmission, as the movement does cause wear to the slipper.