Thats a bad sound and would drive me nuts. First I would try the brakes as stated and see if the sound changes at all. Run the bike on the center stand to see if we can get a better diagnosis. Sounds like a wheel bearing to me, but it will be easy to service the entire back end if your gonna go through the trouble to pull the rear wheel, which I definitely think you should. Continuing to ride the bike with a dry, worn bearing can lead to other problems so I would limit my riding until I got it fixed.