Falling debris within wall cavity/void General problems, solutions, information and advice on fixing to walls of all kinds

[MG]

We're recently bought a 109 year old stone house which is generally good condition. However, whilst trying to put up some pictures using medium-size picture mountings and gentle hammering (nothing too strenuous!), there was a sound of falling/crumbling material within the wall This has obviously alarmed us and we have no idea what to do, if anything! Any ideas? Thanks!