The word “clunk” reminds me of a harp I diagnosed. It was clunking between pedal changes. It was perhaps a “thunk.” It wasn’t metal against metal. I finally figured out, after the three top guys from Lyon & Healy looked at it and gave up, that it was only happening when you moved, I think it was going from G-sharp to A-sharp. I therefore deduced that the rod or all the rods were moving at that point and hitting the inside of the column or something like that. Could it have been loose rivets? Perhaps, only it never stopped functioning entirely. This was a Salvi Electra. As far as I know it has never been fixed.