Hello all.
I noticed recently that there was a weird noise coming from the rear of my trunk when driving, and fearing the worst. I started checking over the suspension. Everything looks good, but the noise continues. So today I discovered that I could replicate the noise by opening and closing the rear doors quickly. I narrowed the location down to the rear left (drivers side) of the trunk. I climbed under, and had someone else open and close the door, but I couldn't see anything moving under the car.

Long story short, started taking apart the inside of the trunk, and when I took out the liner, I found a vent. It has a flap that moves back and forth whenever I open and close the door really fast. I'm guessing that that flap is moving because of the pressure change that occurs when I open and close the door. (I only hear it flapping when the trunk in closed)

So I'm just wondering what this vent is. And, I've never heard it flapping before. Is it broken?
I've attached pictures of said vent.
Thanks in advance for any info.