Could be signs of an impending earthquake:
“Earthquake “booms” have been reported for a long time, and they tend to occur more in the Northeastern US and along the East Coast. Of course, most “booms” that people hear or experience are actually some type of cultural noise, such as some type of explosion, a large vehicle going by, or sometimes a sonic boom, but there have been many reports of “booms” that cannot be explained by man-made sources. No one knows for sure, but scientists speculate that these “booms” are probably small shallow earthquakes that are too small to be recorded, but large enough to be felt by people nearby.”
http://earthquake.usgs.gov/learn/topics/booms.php
That’s what I thought of. I sure hope that’s not it.