Whether it is the pressure relief valve, or a crack in the tank, or even a join failure in the pipes - that AIN"T a good sound! Turn it off and call a plumber!
I going to go with what GeorgiaBob says. I can't think of anything good that would make that sound, but can think of many bad things that can.
Until the plumber gets there, I would turn on a hot water spigot and make sure that there is water, and not too hot, coming out and not steam. If steam, I would turn off the power, or gas and get everybody outside and away from the water heater.