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.
You can't hear mains hum without a speaker (or transformer acting as a speaker).
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.
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.