I have the same issue on a 2008. I thought it was the rear license plate as well. That is the exact sound it makes. I don't find mine is at a certain rpm tho, I think it's more to do with the wind. What I think it is on mine is the actually rear bumper, atleast the plastic covering the bumper seems to rattle. If i tap the rear bumper I can reproduce the exact same rattle. I just figured this out, but I have crawled under it yet to see if there is an easy fix.