I am not an acoustic expert but I do believe in resonance as in musical
instruments and drums. I'd think most of the noises came from the resonance
of the wind on the doors and side panels. Knock or tap on a drum or an
acoustic guitar and you will hear noise or sound.. On a vanagon they put
some sort of sound barrier on the roof but why not inside the door and side
panels? The doors and side panel are just large cavities and when wind hit
them they resonate and amplify the noise for us t hear.