I followed the A/C vent DIY but that didn't fix my rattle. Turned out my rattle was coming from one of the metal fasteners on the trim piece. See photo:
One of my metal clips wasn't shaped properly and would have a metal-on-metal rattle when pinched. Used pliers reshape it. Rattle gone! So my suggestion for those doing the A/C vent rattle fix would be to check the metal clips by pinching them and ensure they don't make a metallic sound.