souzu

A type of Shishi-odoshi. The true name of "the thing that goes doink".

Souzu are bamboo tubes with one end closed that are balanced on a fulcrum, and placed at the base of a fountain, such that they gradually fill with water. When the tube is full, the weight of the water causes it to tip over and pour the water out, which quickly shifts the weight back to the closed end, causing it to hit the ground and make a sharp "donk". Souzu are traditionally used to scare deer away from gardens by startling them with the sound, and are a sign of affluence and wealth.

http://en.wikipedia.org/wiki/Shishi-odoshi
http://tvtropes.org/pmwiki/pmwiki.php/Main/TheThingThatGoesDoink