At 12-years, it is probably on its last legs. Your anode rod is probably long gone, and what you are hearing is water caught in a small pinhole through the glass lining of the tank creating a burst of steam as the water gets heated since it is partially trapped there and can't cool off by convection. A delime at this time at that age may hasten its demise. The anode rod is scaraficial...it is more reactive than the walls of the WH, and gets attached first. Eventually, it litereally gets eaten up. If you don't replace it before that happens, it then starts eating up the tank. Once that starts, you're on the course of no return.