[roadracer311]

Solution! (I've also posted this at Renntech.org)

After a year of chasing this sound, I've finally made it go away on my car (2003 Boxster S with the original top).

The sound was exactly as you describe: shirt button hitting on a hollow tube. It was really taking away from my enjoyment of the car. So much so, that I was avoiding driving it unless I could to so with the top down.

I've attached two photos. The fist shows where to look. In my case it was on the passenger side (right side) under the spring loaded flap shown in the photo.

The solution was loosening the torx screw, pushing down where shown, with one finger while tightening the torx screw with the other hand. The position of the head of the screw moved from one end of the slot, to more than half way toward the other end of the slot.

Caveats: I have no idea what this spring loaded slotted thing is for. For me, it made the rattle go away, and has had no negative side effects. But that was only 24 hours ago.