Out in the country in the middle of Texas yesterday. I heard a very low bass frequency. Was not a bass guitar. Was a certain frequency again in the low range. Came on for a few seconds and then again some seconds later for about 15 to 20 seconds. After the long one, I felt a little light headed. I came in the house from the garage and sat down until my light headedness went away. I have no clue where that came from. I looked around, did not see anything that could produce such a loud low frequency like that.