Something airborne came up from behind us while fishing, it sounded like something large and fast but there was nothing visable. There was a loud sound of air moving but no engine noise. It sounded like it flew from the north east to the south west very fast right above our heads. There was no visable evidence that anything was there as all despite being a mostly clear day. I've went over the event many time and cannot come up with an explaination for what happened, I've spoke with the other two people that observed the event and they don't know what it was either.