Deserttoy,
Sorry I can't help you with the flapping noise, but I've noticed (someone correct if I'm wrong) that when people with convertibles drive with the top up and their windows down, the material looks like it's being stretched from the wind turbulence. Especially on the freeway. Can the material be stretched?
When we got our Mustang convertible, I told my wife not to do this unless she was just driving around town. In other words, I try to avoid high speeds with the top up and the windows down. Just my .02. Hope it helps.
