Our house has one of these in every room. After we moved in and started painting each room we weren't sure what they were for, so I painted around them, then painted the screen white (they started out a gross dirty beige color). They drive us crazy. When the wind blows, they rattle. When the back door is open, they rattle. When the furnace or AC kick on, they rattle. So when we had the AC serviced some time ago we asked the guy if he knew what they were for. Apparently when the house was built it had a swamp cooler and these vents were used to draw the cold air into the room. A-ha! We no longer have a swamp cooler, so we no longer need these stupid holes in our ceilings! Yipeee! So voila here is what the hole looks like in the den now.
It still needs sanding but Shawn did a great job, I am so excited to get rid of the holes!