When I was a teenager in the 80s it was definitely the sound of the rear derailleur being picked up by the spokes. As somebody above indicated, it gives you time to think "oh no" but not enough time to actually do anything about it.

Huret/Raleigh Eco mechs (did you get them in the US?) were notorious for it. Adjusting the stop screw seemed to give you the choice of not getting first at all or firing the whole lot into the spokes with nothing in between.