You have a worn pulley. Turning the wheels makes a slight change to the balance of the engine, which in turn makes the belt squeek against the worn pulley. Since there are a lot of pulleys, you may have to get real close to the noise to determine which one is bad.
Sometimes, you can get the "stop squeek" stuff at the auto parts store and it will temporarily stop the noise. Eventually, though, you're going to have to track down the bad pulley. It may wear down the belt, and when the belt breaks, it usually causes other damage.
Good luck!