I'm curious if you were ever able to reproduce this particular anomaly? Just for kicks I tried it on a system that was in a music store and sure enough... "tick... CLACK". They were quite surprised to hear that as well! The more I listen to it is sounds like a yardstick falling over or a conductor's baton being struck.I'll hold on to my Steinway with a protective clutch---maybe it's a freak copy which somehow turned out perfectly!
Anyhoo, the reason I'm even posting this is because I had actually forgotten all about this (haven't been playing GAS lately) and yesterday I fired it up again for the first time in a long while and was playing away when out of nowhere came that "CLACK" and I had to smile!