I finnaly found one of those Thunk Tunk type sounds to be iminating from my freewheel - Its a long story that involved the systematic upgrade and repair of vairous parts of my 80s UNIVEGA - Each adjustment resulting in eliminating one particular sound but the most anoying and fleeting sound was as yours - Not my casset but my freewheel - I just changed it out for another and have not isolated just why it was making that sound - I suspect that some of the small bearings in the free wheel have failed or gauled...
Hey good news is that I have been over my bike in explicit detail and now it rides smooth as real butter...
Tuto Ballencia...