I have several kitties that make that strange sound when watching birds or squirrels! My mom and I call it "breaking the neck" sounds. It isn't quiet chattering, but, close to it.

Allie does the cry that turns into a yawn, and it is adorable.

Georgie does that "let me down" noise, where she doesn't open her mouth, it isn't growling, but, it is the noise that shows irritation. He he he..I love making her do it.