It's probably not the parcel shelf that's making the noise but the C pillar trim, a loose jack in the trunk, or some other item banging around in the trunk. The pillar trim, in particular, has plastic clips that can break when the trim is removed. Reinstalling the trim will keep it in place, but clearances will make it rattle. It's also much more likely that someone removed the trim to access something underneath it and broke it, versus removing the parcel shelf.