As has been said, you'll have to try to work through things one by one; to eliminate them. Electronic hum ( your box or other electrical equipment ), your heating ( pipes humming or water pressure variation ) and also ( and I'm not kidding ) wasps or bees trapped in the roof space / rafters. We've had problems around these parts, this past several months, with wasps and bees in houses........it's been an unusually bad year apparently.