Grinding/ratcheting noise on left rear wheel area at low speeds?

I recently had the sensor "hub" on my left rear tire replaced because of a bad ABS sensor. About 500 miles after the repair (and into a vacation) the left rear wheel area started making a grinding/ratcheting sound when braking at speeds below 10 mph and when idling forward without the brakes applied. The rotors and pads have ~25000 miles on them, a vast majority highway/ interstate miles. The mechanic who did the initial repair said he cannot tell what the problem would be without seeing it, and I cannot bring it to him until Monday when we return. We have put about 1000 miles on the car while it has had this problem, and we have had no issues stopping. We have had no issues with the vehicle tracking to the left or right, and we have not noticed any problems other than the noises. The brakes are as responsive as they have always been. Does anyone have any suggestions as to what the problem could be based on the description described above?