Are you talking about a vent cover like Maxi-vent or the standard vent cover. If its the standard vent cover I recommend you close them tight when traveling if you are referring to the Maxi type cover they are held on with clips I have never heard of one rattling but I guess it could a little dab of chalk in several places around the edges would stop that.
Jake my sidekick (yellow Lab) 10/04 - 05/20
2017 RAM 2500 CC 4X4 Cummins Diesel
2016 Flagstaff 26 FKWS
AF&AM & El Korah Shrine of Idaho