OK, opened it up, obviously simple. As far as I can tell, there was nothing at all out of place in there. There were no apparently loose screws, no scratches on the inside of the caseback that would indicate rubbing, and the rotor looked fine, spun easily, and didn't appear to be rubbing on anything. The whole thing looked very clean. The "rattling" sound was very reduced without the caseback as well, so I guess its (1) watchsmith locally for an expert opinion and then (2) depending on that, back to the TD. I assume that if I sent it for service to Vac (who is in Florida, like me), he would take care of that issue anyway, so I guess that's a third option. But if the sound is from the rotor making even slight contact with the caseback, I don't see how that could be fixed without either grinding down the caseback, or using a slightly thicker o-ring, maybe. Hopefully it wont come to that!