Another weird thing happened! I backed out of the garage, and then when I began rolling forward downhill there was a horrible metallic scraping noise like Godzilla's fingernails on a chalkboard. I stopped immediately and proceeded to see what it was by process of elimination. I concluded it was in the right rear wheel. It sounded brake-related. It only happened in the forward rolling direction, directly in sync with the wheel rotation speed. The noise was awful. I felt around the rotor and brake pads (fortunately the 17" Boxster wheels give easy access through the spokes, although it is still impossible to see anything except the open outer face of the rotor). Nothing there. Filthy hands covered with brake grime. In a hurry. Worried about having to call for a tow and miss Thunderhill for repairs.

Last resort: ran upstairs to get a can of "computer cleaning" compressed air. Inserted the tiny tube in various places around the rotor and brake pad, and sprayed a jet of air all around in there.

No more noise! What a relief. There must have been some little thing wedged in there somewhere, but I can't figure out when it could have happened, because everything was fine when I pulled into the garage this morning. Then suddenly the noise appeared later in the day upon simply backing out of the garage.

[ I have since learned that this is the classic behavior of something really simple and rather harmless: a pebble lodged between the brake disc and the shield plate. ]