Welcome to the "A" world and to The Barn. Not easy to diagnose without being able to see it. But, a couple of things come to mind. There is supposed to be a hinge gasket at the top of the windshield and if missing could cause a rattle. Also, the screws that hold the windshield up against the wood header could be loose. (either just loose or possibly due to rotted wood) Does it seem to rattle with the windshield closed as well as open? When open the weight of the windshield is pulling at the top more than when closed. Be sure to keep the thumb screws tight on the swing arms to help support the weight especially when the windshield is open.