Earth’s background free oscillations (a.k.a. Earth's Hum and The Seismic Hum) were discovered in 1998 using a superconducting gravimeter at Syowa Station, Antarctica, and were subsequently confirmed by more than 200 other experimental sensors worldwide. (Ref. Kiwamu Nishida, University of Tokyo )

The oscillations, which tend to change their frequency (i.e. 'pitch') and amplitude (i.e. 'volume') over a few days, are predominantly in the waveband of 2 to 20 milli-Hz i.e. 1.2 to 12 cycles per minute.

The waves, which have been found to be persistent over the years since their discovery, are measured by gravity meters which pickup tiny variations in gravity (i.e. caused by variations in density/mass). The oscillations were originally thought to be 'echoes' of earthquakes, but this has now been ruled out by correlating the waves with records of earthquakes.

The source of the oscillations is still not clear, but a theory suggesting that they may be caused by atmospheric loading (and possibly oceanic variations) was introduced in 2002.

See : A theory of the Earth's background free oscillations Journal of Geophysical Research: Solid Earth, Volume 107, Issue B9 p. ESE 11-1-ESE 11-10