Rear brake noise

My 2013 has developed a noise in the rear brake when stopping. It occurs only at very slow speeds just before I come to a full stop. It is a pulsing gridding noise. It only occurs after you get the bike to a riding speed and stop, not if you are at a stop and try to reproduce the noise without getting up to a higher speed again. I had a new rear tire mounted recently by the dealer, but I have ridden it a couple of times since replacing the rear wheel on the bike, but I first noticed it yesterday. My first guess was bent rotor from the tire change, but it does not show any obvious wobble and I would think that the noise would have been there on the earlier rides. My second guess is a sticking brake shoe or maybe the rods the brake shoe rides on have developed a pit on them from my testing the braking of the new tire. I took to the dealer yesterday and the mechanic said that the brakes stop fine and that maybe the shoes have not seated in yet (2,200 miles on the bike?) or some dust or a small stone had gotten into the shoes and/or caliper. I am going to remove the rear wheel today and see if can see anything. I am going to take back to the dealer if I can not resolve it myself. I am leaving for a 8,000 mile trip in 10 days and would like to have it resolved before I leave. It would not be good to have a problem on the road.

Any ideas or suggestion?