I'm looking for data on "acoustic noise" produced regularly by the earth, nothing man-made. In particular I'm looking for noise sources outside of the normal range of human hearing (20Hz-20kHz). Of course there are numerous terrestial noise sources, such as earthquake tremors, but those aren't predictably regular. To be even more specific, what I'm really looking for are noise source candidates for terrestially emitted "acoustic zeitgebers". For those whoe aren't familiar with the term zeitgeber, it means time-giver. It's commonly used in the field of study of biological rhythms. A good example of a non-acoustic terrestial zeitgeber is the daily sunrise/sunset transition that sets the body's circadian phase. So, in summary a candidate for a terrestial acoustic zeitgeber would have to be a "noise" source produced regularly by the earth in a cyclical fashion (though not necessarily daily). Again, if such a zeitgeber exists it's probably outside the range of normal human hearing or I'd most likely already know about it. Thanks in advance to any who can help!