You can't hear it, but the Earth is humming to itself at a seismic eigenfrequency of about 10 mHz and with an excitation energy equivalent to a magnitude 5.75 earthquake. There has been controversy in recent years as to whether atmospheric turbulence or ocean waves were driving the hum.
Now Spahr C Webb of Colombia University in New York has calculated the coupling between ocean waves and seismic modes and found that it reproduces the seismic spectrum of the hum. Interestingly, the mechanism depends on the weak nonlinearity of ocean waves and the interaction of waves with the shallow continental shelves.
Further reading
Spahr C Webb 2007 Nature 445 754.