I have recently encountered an unusual cause of hum.
For the last couple of months I had a low level buzz/hum develop. However it was loud enough to just hear at the listening position in quiet passages.
I have been frantically chasing this.
Last weekend it started to fade progressively and this weekend has disappeared entirely.
Now we have had a severe drought here that started August 15 last year.
We had very little snow and NOAA have had us in moderate drought.
Everyone has been frantic about forest fires, and we had one two weeks ago at the town of Nimrod about 30 miles from here, that destroyed 1600 acres, eight structures, including one home and the National Guard was called out.
Last weekend we had torrential rain, ice and finally 5" of very wet snow, that took two days to melt.
Last weekend as we received two inches of water as rain snow and ice, that hum/buzz gradually faded. Yesterday it rained all day and most of the night. Today the hum is gone.
Coincidence? Not likely.
Despite my uber ground of three seven foot rods driven to the hilt into the ground and tied together, the soil must have dried out to the point were the house grounding was not adequate.
So in future when it starts drying out I will give those rods a good watering. I will water them well before putting the hoses away prior to freeze up.
I never thought the water table would recede bleow those grounding rods, but obviously it did!