I'm pretty sure it's wanting to eat them. My cat makes the same sound when looking out the window at the birds on the deck. I read about it in a book and it said something like "Cats make this sound while hunting." which makes no sense to me, since the point of hunting is to be quiet isn't it?
Basically I think it means: "I'm nice and totally not going to eat you. Look, I sound like Chewbacca, that's cool right? You wanna hang out and totally not get eaten. You can work on your ArTooDeeToo impression. And did I mention that I'm totally not gonna eat you?"