When I went outside, it did not sound like anything I had heard or felt before. It was bigger than the really loud man made stuff we just ignore in life.
After I could not dismiss it as a very loud train (because I am not near any trains), I told myself at the time that it seemed to be coming from the heavens...but I have no clue what it was, really. It did not seem to me at the time to be coming from the earth. Others around here heard it as well. My husband heard it at work and told himself it was a really loud ship (he never hears ships at work! LOL).
I played one of the videos again and my Cockatiel got distressed to the point I would have to take her to a different room (if it went on any longer).
Sara - these two Freepers have heard it too.
2nd Div and dForest, you might compare notes with Sara. Also, note the comments about the dogs reactions to the noise.