The Earth vibrates at a discrete set of frequencies, as discovered by Nawa et al in 1998. These frequencies are much lower than we can hear, between .002 Hz and .007 Hz (we can hear in the range of 20 Hz to 20,000 Hz). There are also microseisms, much higher frequencies (a whole .2 Hz, still 100 times lower than we can hear). Seismologists have known that the higher frequencies are caused by ocean waves interacting with each other, trasmitting pressure fields through the depths of the oceans to the floors below. But now they think they know why the Earth hums at the lower frequencies as well. Winter storms in the oceans (north in boreal winter and south in the austral winter) provide the continuous energy oscillations that make the Earth ring like a bell. Still no word on when the Earth will be coming out with its first CD.
Barbara Goss Levi, "What's Making Earth Hum?" Physics Today (March 2005), 22-23.
K. Nawa et al, "Incessant excitation of the Earth's free oscillations," Earth Planet Space 50 (1998), 3-8.