Arrived onsite for a rattle from a supply register vent. Performed diagnostic. Went in attic and look at the register. Heard rattled there too. Did not find anything on the outside of the register that would cause a rattle. From the bedroom; removed the register cover. Rattle went away. Played with the vent damper by opening and closing it. Replaced register cover. The sound was not there. Observed for 10 minutes to ensure the rattles did not return. System working as it should.