The low hum is probably the circulation pump and not the ozonator. The ozonator comes on with the circulation pump, but nothing to do with the heater. A UV ozonator does have a Ballast(Transformer) and on some rare occasions can make a hum, but not too likely. The constant sound or low hum, is what you would expect to hear from a circulation pump. It should not be keeping you at night, but it can be heard. Dave.