If it bothers you then you will have to de-mount the tire, as it's not practical for us to just guess at what it is. As with any diagnosis you first need to test all the variables. First note about where on the wheel the thump occurs in releationship to the valve stem. Remove the tire and look for any difference in the tube or tire at that point. If you still can't determine the cause mount a different tire (or switch with the front) but keep the tube the same. If the rear still thumps then try a different tube. I would think that should do it.