I don't know if it's the wheel or the tire, but I had the same problem with my front tire a while back. I did basically what alumrock said, i.e., I just deflated the tire, removed and reinserted the tube (after inspection) and inflated it. It worked good as new. Hopefully that'll be all you need to do. I could tell that the problem wasn't the wheel without having to take the tire off, however, because I just held a screwdriver close to the rim and spun the wheel to see if it was out of true, which it wasn't.