I have a bathroom exhaust vent with louvers similar to the image below. It vents out the side of the house above the bathroom between two bedrooms. When it's breezy out, the flaps rattle so that it sounds like we've got critters in the attic. (It was actually pretty freaky to hear, since we had just moved in.) The bathroom fan is never (rarely) used, so I'm certain it's not the cause of the flapping.
So - is this normal and what's a recommended (quieter) alternative which would provide adequate exhaust without allowing pests to enter?
Sub-question - Last year, I blew in 6-8 inches of insulation in the attic. What might be the likelihood of replacing the vent just from the outside without needing to trek up there and disturb/compact all my hard work?