A rather clever device from Japanese architecture. Basically, a floor built so that it 'sings' when walked across. This was intended to warn the building occupants that an intruder was up to some skullduggery.
An example photo can be found here:
http://www.johnharveyphoto.com/Japan/Kyoto%20Day%201/NightingGaleFloors.html
Sources:
http://www.kansai.gr.jp/culture/build/archi_e.htm - This site has some sound files of what the floors sound like.
http://www.probertencyclopaedia.com/F5C.HTM