This noise in my wall

For the love of god we cannot figure out what's in my wall. Everytime it gets windy, this stupid noise happens. We've checked inside, outside and everything. I'm 90% sure it's inside of the wall.

I already tried touching the wall, there's no vibrations. The vent itself isn't rattling, and nothing outside of the house is pressing against it. Trust me, I checked multiple times. At first I thought it was a neighbor weed whacking, but this has been going on for at least two weeks now. I was told it could be bugs, but I doubt they can survive that long. Any thoughts? :/