My crank produces a quiet kind on clunking sound when rotated after standing on a pedal in the 6:00 position. The cluck happens in a particular spot in the rotation and is both audible, and I can feel it as an interruption in the crank's smoothness. The rotation does not to be under any excessive load when rotated. Does anyone know why its doing this? Its a dura ace crank from about 2004 on an aluminum bicycle.