Okay, so I re-seated the rear wheel in the rear dropout, checked the RD hanger (it was not perceptibly bent), and re-tensioned/adjusted the rear derailleur cable.
On our ride this morning, the grinding sound was still there, but it was much quieter. This tells me that either (a) re-seating the wheel helped, or (b) the RD cable adjustment helped. Progress! I will continue to play with the cable adjustment, and give all relevant parts a good cleaning (the bike is due for one anyway), and hopefully the grinding noise will continue to subside.