"We know that the whole creation has been groaning as in the pains of childbirth right up to the present time." - Romans 8:22

More mysterious noises emanating from the earth... this time not in the Midwest, but in California.  Aircraft, mining, and winery "cannons" ruled out:

http://www.dailymail.co.uk/news/article-2193754/El-Dorado-County-California-residents-baffled-mysterious-thunder-like-sounds-theyve-hearing-YEARS.html?ito=feeds-newsxml