Sorry to take so long responding - I didn't know there had been another post re this question.
Noise is not related to rpm. It is not a periodic noise, like a scraping pad on a disc with runout that has an actual frquency. It just happens once when certain things are done with the car: shift into first after it has been sitting at least a couple of hours; shift from reverse to neutral after sitting (cold); coasting to full stop in the first few minutes of driving; stepping on the gas from a dead stop or low speeds after taking foor off gas and coasting. When car is thoroughly hot, as after 50 miles at 50mph or better, there is no noise at all. All of the above may sometimes occur under the conditions noted, sometimes not. Perhaps a coincidence, on Saturday (not before or since), the parking brake light did not go off after releasing brake for about 20 minutes of high speed drive. Later same day, remained on at startup, went off in a few minutes, went back on for most of 40 minutes, then went off. Brakes did not seem to drag, rear discs did not get hot.
Does this make any sense?
Thanks for any light you can shed on this.
One more thing: when car is cold, starting up but shifting from neutral to drive with foot off brake and also off gas pedal, the "toc" noise is clear, car moves forward slowly, hesitates, noise happens again and car moves forward smoothly at slow speed. Like something trying to engage and slipping just one time. This does not happen later in the ride.