If you have double wall aluminum rims, it's possible that a small bit of debris broke off and is now rattling round inside the rim. This is very common, and you can hear it rattle if you take the wheel off and shake. However you rarely hear it while riding at anything over a few miles per hour, because the aster spinning wheel forces it out and holds it against the outer wall.

Shake the wheel and see if this sounds like the problem. If so, you can sometimes shake whatever it is out a spoke hole, or the valve hole if you remove the tire. Otherwise forget about it.