Back in the 50's or 60's (could have been earlier or later) I believe as a part of switching to the radio clock and having a standard that everybody could utilize, the absolute zero of the clock was used to sync all the AC generation in the country. This led to some unusual phenomena such as pulsing of the earths flux field and was discontinued, effectively eliminating the unusual pulsing of the earths flux field. While the pulsing was not something the average person could detect it was noted by some that monitor such things. The pulsing was of course low frequency at or about the sixty hertz level.
Sounds very Buck Rogerish, but it is a science that might explain the odd humming heard by some folks and not others. Certain elements and crystals vibrate at different levels when exposed to varying levels of electricity. The density of the earth's flux field varies and may cause the vibrations when intersecting with the plains in certain crystals. There were some articles in American Scientific related to the phenomena.
