Shut off the power to the hot water heater wait 5 to 15 min. If the noise stops it is the water heater eating up electricity. Turn it back on if noise starts again feel about 5 or so feet down the hot water line. If this line is hot you have a leak beyond the hot water heater. If you still hear it when the power is off to the water heater. next turn the water off on the cold water line at the water heater with the power off also Look at the water meter and see if the hand or wheel on the meter is moving and it should no be. Also think if the breaker to the hot water is off the electric meter should practically stop if no refrig oe anything else on. Have someone watch the electric meter and switch 1 breaker at a time leaving the one you just shut off in the off position until you cut them all off. Should the meter still be moving you have a main service cable shorting somewhere call the power company to advise you further if this is the problem.. Should just cutting the circuit breaker for the hot water heater stop the noise and down the hot water line is not hot then you are probably wasting power due to a thermostat or element inside the hot water heater.. Thank you j