Update: Took it to the shop for an oil change and to get a professional opinion on the noise. Their initial impression was the same as yours, that it was probably a rock between the brake and the backing plate. However, when the pulled the splash guard off the bottom to change the oil, they found the crank pulley nut sitting on it, and said it looked like it had worked its way off and rubbed against something. The good news is they got that reinstalled and torqued down properly, and checked the accessory and timing belts and didn’t see any damage from them having a bit too much freedom.
Scraping starting around 0:03.
(Caution: may be a little loud, the dashcam didn’t pick it up too well, so I boosted it, but there is a bunch of background noise as well)
Oil is low, but there. Don’t think I was on the brakes, since I was going uphill. This is the only time I’ve heard it.