When we first bought our new Class C, our dealer told us that was normal and sold us a $15 pillow designed for the purpose. It works and it also helps to keep the cab over area cool. All of our vents rattle a little at highway speeds. Fortunately we don't hear the others. One day we'll actually get MaxxAir type vent covers, but until then, the pillow works just fine.
Here's one on
Amazon. We have the one without the reflective surface. I don't know that you'd want the reflective surface. If the vent is closed all the reflective surface would do is bounce any UV rays back to the vent cover, possibly shortening its life.
-Michael
-Michael