Several threads exist on this topic. There is the possibility that hum is being caused by DC on the AC power line. This can come from a number of sources, including light dimmers, and certain devices with speed and temperature controls. Do you have any of those devices in operation at night ? Try shutting off breakers one at a time to isolate the source. It is also possible the source of DC could be the house of a neighbor who is on the same power company transformer.
