I recently noticed that there is a high pitched noise coming from my water heater. I have an Atwood 6 Gal. water heater. The noise disappears when the water heater is turned off and reappears when the water heater is on (both electric or propane). The water heater functions normally. The water gets hot on both electric and propane. There are no leaks. I've had the circuit board swapped out, but still the noise persists. Does anyone know what could be creating the noise?