Thanks Jason! I have heard it a few times since and agree that it's probably a gap in the weather stripping somewhere... I noticed today that the passenger side door sticks out a little in the top-back corner, I'm assuming this is where some turbulent air could rattle around causing the noise. I'll try to DIY with some weather stripping
You'll see this in other threads as well, but this stuff seems to work well. Sealed up my 3rd brake light nicely as well (better than the factory seal up there), no leaks yet.