If the spokes were poorly tensioned enough at one time they could have flexed enough to rub at the cross, creating a burr that makes noise even with minor flexing. Take hold of the spokes just toward the outside of where they cross and flex them in and then out - see if there's resistance at one point. You may even dublicate the sound. If that's the case you can smooth out the irregularity - sometimes a little lubricant helps as well.