One of my cats makes that chattering noise, too! He never wants to go out, though. The other one, he likes to escape outside, but he always turns around and comes right back in. It's like he gets a good look at how big everything is out there and decides, Nope!
We have had 2 incidents though, where a bird has wiggled their way into the house where the air conditioner is. I don't know how they get through there, but it's happened twice. Oh boy, did they ever go nuts! The first time it happened, they chased the bird down into the basement, and we were hot on their heels, trying to save the bird. We ended up losing it for about 30 minutes in my library, but it was hiding up on top of one of my bookcases amid all of my bears. The second time it happened was just a couple of weeks ago, during that really, really cold snap we had, and I think the poor bird just was too cold to fly, he was easy to catch and let back outside before the kitties got him.