I had the same problem on a yr2000 883 hugger. Just after riding out a couple of miles, it will squeak really loud. As Sundog mentioned, I went to get the bearings checked, and it was out of alignment. After changing it, the sound disappeared for a couple of days but then it returned again. The problem actually was with the pulley, as the teeth had somehow worn off. I had to change the whole belt drive system, including sprocket and belt. After that it became alright.
To temporary stop the squeaking sound, what I did was to rub a cake of soap (dry) on the belt itself. I went through 2 cakes of soap before the problem was solved.
Hope yours gets rectified soon.