I think I know what your problem is! My guess is the trim moulding running along the top of the windshield is coming out, and at speed, the airflow is making it vibrate.
When I got my KJ there was no moulding. I thought that was odd, because it looked unfinished, & dirt collected there. But my Jeep experience has tought me to expect things like this, so I thought nothing more about it.
We had the windshield replaced a couple of months ago. I didn't think about it until a couple weeks ago when I was driving & the noise started. I pulled over, & the moulding had come out in the middle, & was slapping the car! I snapped it back in, & a week later, it did it again.
My thought is that the original had done the same thing, & the previous owner just got tired of dealing with it, & pulled it completely off.