I'm unsure of why they do it, but it seems that all cats do - Maybe as a signal to other cats around that they are hunting something and not to go for it??
When a cat kills prey, it bites down quickly several times. The "chatter" is simulating the cat's behavior at the time of attack.
Alternatively, it's pure and simple frustration. When a cat is stalking or killing a bird, they're almost invariably completely silent. It's when they are shut off from the prey that they chatter.
Alfie does this when we play with the laser. He'll chase it around like a loon, but if I shine it up too high for him to reach he'll just gaze at it and chatter