Curious, is this "mains frequency" the same ringing I often hear when in alleged silence? And, no, I'm not talking about my hearing because I've had conversations with others who witnessed it :P - more like the ring old tube TV's used to make.
I believe that you should be able to hear overtones (2* or 4* mains freq) coming from lightbulbs and power supplies (computers, chargers, etc) in an average room - it could be tested by running a decent mic + solidstate recorder on batteries and checking how loud it is.
That may be true, but there are plenty of things in your home that serve that purpose. Right after reading the first sentence of the article, I started hearing it. Either 60hz or 120hz -- a low B natural. Maybe it's coming from the heating ducts, or maybe it's a fluorescent light bulb. Heck, maybe it's the fridge.
It's really bad whenever I plug speakers into the wall. Not sure if it's the mains, or the radio tower that's 500 feet away, which also "allows" me to listen to NPR at conversational volume without use of a radio.
The brain is designed to process signal and if there is none it will sometimes make up its own. I guess you can compare it to dark current in photo-multipliers.
People who have tinnitus will hear the sound you describe or something similar all the time. When I was little I had the same experience as you, but now I have developed tinnitus and never get rid of that sound any more. Sometimes it also becomes rhythmical which very annoying. The only thing that helps is to ignore it.
You're probably hearing sounds that your body is making. The sound of blood moving in your ears is pretty significant when you're in a silent environment.
