It's intermittent.
Not connected to engine speed.
Usually follows road speed (but not always

Can only be heard at low speed.
Appears to be coming from the roof or high up on the sides.
It sounds like something slapping the roof in the wind, but I've been up there and there's nothing loose or extra. The pop-top clips are all secure and there's nothing in the fishpond (it's a Holdsworth).
I know it's next to impossible to diagnose something like this without seeing it, but if anyone has any, ANY, even the vaguest of ideas what it could be, I would appreciate it.
IT'S MESSING WITH ME MELON, MAN!!!
