One possible theory to the phenomena is the sounds are being caused drilling to make way for large underground bomb shelters as some Governments are taking the threat of Mayan 2012 end of world theory seriously.
Another theory is, as always with unexplained occurrences, extraterrestrial beings or God and even the sounds of sasquatch’s howl.
Others have suggested the strange noises might be the work of High Frequency Active Auroral Research Program, HAARP. HAARP is a US-based research project that studies the ionosphere - a part of the Earth's upper atmosphere - and it has regularly been the subject of conspiracy theorists' discussions.
This video was posted on youtube on April 19th, from someone in Dallas, Oregon. You may want to turn down the volume because it's pretty loud. The video is self explanatory, so enjoy and let me know what you think it is.