[Auf los!]

I'm starting to run top-down this rather cool Spring, and when top-down (and only top-down) I am hearing a curious sound which seems to be coming from the right rear. I've had the car for 6 years now, and I can't recall this particular sound before.

It is intermittent, not rev-related, or only heard when turning or braking. Because of wind-noise, I can only hear it under about 50mph. It sounds exactly like the hooting sound you hear if you blow across the top of a bottle! I'll hear the hoot for a second or two, then nothing for a few seconds, then the sound again. Weird.

Ideas?