Well interestingly it sounds a little mechanical to my ear. However it disappeared when I changed to my new dural sprocket. It reapeared when the sprocket had worn again. Tightening the belt again would probably solve the problem but I don't like running to tight as I worry about bevel box bearings. The local dealer thinks it's the belt possibly slipping a tooth but on balance I just think it's the snap of the belt. Happens if you let any backlash into the drive system.
Typically: overrun, breaking into slow turn, change down without a blip and therefore backlash (slack is maybe a better term) introduced. Open up gently and a noticeable little clunk. Only small but not a great sound. Doesn't happen if you get it right and down change into first blipping all the way and gently power out, but hey it's not always possible in town etc.
