Perhaps this response is similar to the primitive "fight or flight" response. Natural sources of these "infrasounds" include "earthquakes, severe weather, volcanic activity, geomagnetic activity, ocean waves, avalanches, turbulence aloft, and meteors and by some man-made sources such as aircraft and explosions" according to this site:
http://www.etl.noaa.gov/et1/infrasound/
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Perhaps this response is similar to the primitive "fight or flight" response. Natural sources of these "infrasounds" include "earthquakes, severe weather, volcanic activity, geomagnetic activity, ocean waves, avalanches, turbulence aloft, and meteors and by some man-made sources such as aircraft and explosions" according to this site: http://www.etl.noaa.gov/et1/infrasound/