Just got back from a test drive, VR looks fine - steady at 14v on the highway, down to 13.7 with headlights, 13.5 with headlights and AC. No spikes at all, on the highway or city streets. No smell at all. Couldn't get the tire to lock up on downshifts, either going straight or in a curve. Haven't had a chance to jack it up and get under it yet.

Porsches never act up on the test drive!

Any roadkill marks on the tire :D ?

TBH, I'm wondering if it wasn't something like that that just got stuck somewhere, burned up, and is now gone.

Yes. If minimized when double-clutching, the clutch disk, if oil-fouled, will tend to grab instead of typically slip when torque continues from engine to gearbox.

If so, check rear main and/or input shaft seals.

Sherwood

A good thought. If it is the throw out bearing, it will become more obvious as time goes by.

Ok, I'll keep an eye (and ear) on that.

See if you can (re) create the sound other ways to isolate the cause. Or just wait to see if it appears in other scenarios, or changes with time.

For example, if it is the throwout bearing, it ought to make the sound any time you depress the clutch and not just on downshifting. Try downshifting earlier or later than usual to see if the sound changes, or perhaps it may reveal that the timing of the sound with downshifting was coincidental, etc.