[Basically... they don't really know.... but they have a few theories]
We have analyzed records of these sounds and found that most of their spectrum lies within the infrasound range, i.e. is not audible to humans. What people hear is only a small fraction of the actual power of these sounds. They are low-frequency acoustic emissions in the range between 20 and 100 Hz modulated by ultra-low infrasonic waves from 0.1 to 15 Hz. In geophysics, they are called acoustic-gravity waves; they are formed in the upper atmosphere, at the atmosphere-ionosphere boundary in particular. There can be quite a lot of causes why those waves are generated: earthquakes, volcanic eruptions, hurricanes, storms, tsunamis, etc. However, the scale of the observed humming sound in terms of both the area covered and its power far exceeds those that can be generated by the above-mentioned phenomena.
Full article at: Geochange