Simply an harmonic convergence. Sound waves do not dissipate entirely - they reverberate around the planet similar to sonic and subsonic particles. The human ear is only capable of detecting certain levels of wave-lengths - that does not mean the noise does not exist.
With cataclysmic events - earthquakes, tsunamis and man-made noise, radio transmissions, explosions, energy production etc., there are instances when all those sounds have to merge or avert.
Such a cacophony can be heard, on occasion when people are in elevated, isolated areas and turn off their cell phones.
http://whatis.techtarget.com/definition/0,,sid9_gci900512,00.html
For those who were in Northern California that day, a deep, muffled roar could be heard a few minutes after 5:00 p.m. on October 17, 1989.
It sounded like a distant but powerful freight train. For those who remembered the Alameda Naval Air Station, it sounded like fighter jet aircraft taking off or landing.
The source of the sound, of course, would become apparent moments later.