cracked plate?

[J Patrick Draine]

>     Heard (and felt, through my tuning hammer and the floor, as 
>though someone had dropped an armchair or sofa) a loud bang from a 
>1903 Rembrandt upright I was tuning today.

Did you check the frame thoroughly? Maybe the glue joint of one of 
the support beams gave way. If so, it's probably a lot more 
repairable than a crack in the plate. Good luck,
Patrick