When experiencing boiling noise, turn off power! Open the water heater, carefully remove one set of wires, probably the lower element is a better choice "Make sure exposed wires are well away from anything, keep kids away" turn power back on, see if you get the same noise, if so its the top element, if not its the lower element. Possible causes are build up of Calcium sludge with hard water, damaged elements or the wrong length of elements. One can get a 3 kw element in several sizes and lengths.