I second everything Jea48 has to say.
I would put not just switching off but *unplugging* everything close to the top of your list (off sometimes isn't really off). The next time it happens, have someone stay near the system listening, while you work round the house unplugging everything (everything--even the fridge). If the buzz suddenly goes away, they can shout out and you can figure out what was injecting nasties into the electrical.
Another thought. There are inexpensive electrical testers (looks like a 3-prong plug with several lights on it) that you can go round plugging into all your outlets. The lights light up in a certain way if all is well, and light up in other ways to indicate a variety of faults.
Good luck.
I would put not just switching off but *unplugging* everything close to the top of your list (off sometimes isn't really off). The next time it happens, have someone stay near the system listening, while you work round the house unplugging everything (everything--even the fridge). If the buzz suddenly goes away, they can shout out and you can figure out what was injecting nasties into the electrical.
Another thought. There are inexpensive electrical testers (looks like a 3-prong plug with several lights on it) that you can go round plugging into all your outlets. The lights light up in a certain way if all is well, and light up in other ways to indicate a variety of faults.
Good luck.