I have not investigated the material from which the "slide out" roof is made. All I know is it sounds like being inside a drum in the rain.
Retract the slide out unit and the noise goes away.
I'm not going to the expense of having a slide topper installed. I'm sure it would require drilling into the wall of the travel trailer and into the frame of the slide out unit.
Don't want to do that.
I've read a lot of posts to the "Open Road Forum" (different website) with a lot of accounts of "slides in" and torn toppers when the wind comes up (typically beach camping).
I'll use a soft closed cell foam float pad with a cargo net and bungee cords to hold it down if it looks like rain.
Or like mallard29s suggests, just don't camp in the rain. (I guess that's funny to some people.)
I prefer the K.I.S.S. solutions.