Boom

A Boom or Boomer is a large scale sound-wave that appears in certain locations for little or no reason. A result of Spectral Phenomena, a Boom is normally harmless however there are cases of people being injured in the concussive sound wave if they are too close to the point of origin for the Boom.
Booms are usually one-time occurrences in a given location, although there are places around the world where multiple Booms have been heard over a span of decades. Materialists scientists have researched these occurrences extensively and are left baffled. 
References:
http://strangesounds.org/
http://news.discovery.com/human/mysterious-sounds-reported-around-the-world-120208.htm