What's this new starter noise?
 

While trying to adjust my WUR/mixture to accomplish smoother starts and warmups in colder temps I have been starting the car a lot more and more frequently than usual.

Lately a couple of times the starter has turned but has not seemed to engage, resulting in a ZZZZZZZZZZZZZZZ kind of sound and the engine not starting. Another turn of the key will start the car, but I am concerned about this new noise.

Is this a worn gear, or could the starter be loose or misaligned somehow? Or the flywheel?

This happened when the engine was warm but not hot, about 180F.

Thanks.

911 engines generally stop in one of three places. if you look at a starter/flywheel ring gear, those places have much more wear on the teeth that the other spots. so i suspect you have a patch of worn teeth. given enough grinding, the starter teeth will get worn too.