Some Mavic rims click. The ends of the rim are plugged over an internal piece of aluminum before welding and machining, and the rim's crimped to keep that internal "plug" from rattling, but sometimes it does anyway. In that case, one straightforward way to immobilize and silence it is to get a sharp nail and hammer, and use the nail as a punch to crimp the rim better. Another method is to get a can of expanding-foam house insulation and inject the rim through the closest nipple hole, letting the expando-foam immobilize the plug.