Recently I took a fairly long trip from Illinois to Tennessee to pick up my daughter's puppy whom we were sitting for a week. I had no problems on the drive down to Nashville. It took about 4 hours. I had lunch with my daughter and started back to Illinois with the puppy. About an hour into the return trip a loud buzzing sound started coming from under the dash on the passenger side of the truck. I was confused as to what it was. I tried several things with no change but when I turned the head lights from auto to normal the buzzing stopped. I turned them to auto and back to normal several times. Each time the buzzing returned when set to auto. I left them off auto for about an hour and when I switched them back to auto the buzzing never returned.

Does anyone know what the buzzing might have been?