"Scientists believe they have pinpointed the origin of a low frequency "hum" that emanates from the Earth. The noise - which can be picked up in the two and seven mHz (millihertz) range - occurs far below the threshold of human hearing...Both sets of measurements agree that the "hum" originates from the northern Pacific Ocean during the northern hemisphere winter and the Southern ocean during southern hemisphere winter...These locations also correspond to the regions of maximum storm activity in the northern and southern winters. Rhie and Romanowicz think some of the energy contained in powerful waves generated by ocean storms at mid-latitudes is transferred via infragravity waves to the seafloor. Infragravity waves are probably generated in shallow water from more common types of ocean wave and are indirectly driven by winds over ocean basins. On the seafloor, these are converted to seismic waves and it is these that generated the hum."

The BBC News/Nature article may be found here.
Looks like our (water) planet isnt' so quiet after all - a "common sense" science report.
Enjoy, Vince