Monday, January 7, 2013 6:17:12 AM
Its called the liedenfrost effect(see video below), the noise is when the effect wore off and the nickle began to cool rapidly.
Sunday, January 6, 2013 10:30:48 AM
It seems as if the nickel being so hot made a field of heat around the water and the water cooling it over time at the rate it was cooling was "pricing" layers of heat eventually hitting the surface and rapid cooling causing vapor heat to expunge profusely ending. The ping noise I cannot guess.