Another writer suggests VLF from TACAMO airbourne tranmitters could be the cause:
http://www.scientificexploration.org...8_4_deming.pdf However there is little evidence that people can directly perceive VLF or any other radio frequencies.
Assuming the phenomenon really is external and accoustic, my pet theory is that it originates in the oceans, where there is a mass of millions of tons of water in motion, with the resulting energy transmitted to the ground (sea bed and/or shore) possibly causing vibrations which due to geological conditions may be amplified and detectable at the surface at considerable distances inland
SW