OK, I went through the tracks off the gospel album I digitized for a friend. You can hear it distinctly in the right channel 3 secs in.
I just got a remote read gas meter last month, and that is the only remote read device I have. The sound (and recording) predates all that.
I even here it through the amp connected to my computer (obviously you really only hear it when not playing anything). The setup is all run through a decent, grounded UPS. Sometimes, you hear it a lot more than other times.
While I and others have heard this sound in their homes, too, the only single device I have every been able to say I think it is coming from is a transformer for some low voltage under cabinets halogen lights. When they are on, I can hear the sound in the general area where it is.
But, again, I have heard this in an empty house without even curtains on the window!