This phenomenon really has me intrigued as well. Here is a Coast to Coast broadcast where Linda Moulten Howe goes into detail about it, with some interviewing of witnesses. Show aired about 4 months ago.
http://www.youtube.com/watch?v=WbhEANK3iz8&feature=channel&list=UL
Very fascinating indeed, what is this sound?! How haunting and somewhat unnerving. Few different varieties of sounds as well. Interview is good, highly recommend checking it out for people interested in the subject.
Peace. ~
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What are the stars, but points in the body of God where we insert the healing needles of our terror and longing? - Thomas Pynchon, Gravity's Rainbow