This is regardless of source. It is independent of cabling, pre-amp and amp-driver tubes, all of which I've swapped out to no avail. It remains the same amplitude regardless of volume setting. The only constant is the timing of it. Starts and quits every night at the same time.
The stereo is on a dedicated 20-amp circuit and my appliances, when functioning, don't perform with that regularity.
I'm guessing RFI, but there are no hams in the neighbourhood and the nearest cell tower is several miles away.
If so, how would one diagnose and eliminate it? Or could it be crawling out of the power xformer on the pole? Someone had suggested street light noise, but the ones around here start up and quit with the daylight, which being near the 49th Parallel means vastly different times, so I've ruled that out. Ditto the next-door house, with whom we share the power xformer; the house has been empty for over a year due to a bank repo.
What could one do to diagnose and address this? Anyone experienced anything similar?