There is some wind moving around in the area behind those speakers, as evident by the center speaker panel popping out (for those of us without speakers there), when exceeding the speed limit. Mine usually popped out around ~110-120mph and went flying. I stuffed some polyfill in there to limit the airflow, you may want to do that behind those rear-side speakers as well. I'll have to pay attention to any specific sounds coming from that corner, as it would seem louder just due to the tire being closer to your ear.
I'm also running a solid plexiglass windscreen that may be affecting that as well.
I'm also running a solid plexiglass windscreen that may be affecting that as well.