It has nothing to do with the opening and closing if the vents. Whether opened or close, you can hear the AC and it almost sounds like a hotel unit. After owning many vehicles, I‘ve never heard this sound. I am under warranty so I will take it to the dealer.I'm guessing you're talking fan noise. If you close vents the fan has to work harder and it becomes louder. Set your desired climate control temp high enough that you don't need to close vents and it shouldn't be too loud. If course, loud is a subjective term!!!