I'll have to put more here later.  We have quite a bit of preliminary data but not yet in a good form for the web.  In short we have found that when a soft, (but not too soft) plastic ball filled with water is dropped it can sometimes be made to ping fairly loudly with an almost metallic sound.  It is almost certainly some kind of resonant sound wave in the bulk of the water but the details of how, when and in what form it comes about are still being ferreted out.  Three of our students have presented preliminary results at both Research Day at OSU and at the AAPT meeting at ECU last semester.  Until I get more info on this page if you want to know more just contact me or see Gregg Falling, Alex Loch, or Ezra Cole for a personal demonstration and discussion of what's been found.  We hope to be doing quite a bit more with it this semester.