It’s like the cat version of the liquid-metal Terminator going into the pool of molten alloy.
My mom’s cat does that when you scratch a spot on her (the cats’s) back, just above the tail. We’ve always wondered why; she can’t stop and will keep doing it as long as you scratch the spot.
My cat Joanna used to do this. Usually when she’d be standing on the window sill, looking at the birds.
The behavior is actually one of the things that made me take her home. I was at the local pound and an eager animal tech saw me looking at her. He said, “This cat is neat. She does a cool little trick.” And he took her out of the cage and put her on a nearby window sill. Sure enough, Joanna began to make her little noise. The fact that she did it on cue made me fall in love with her.
I miss her! 
I hope that cat’s been checked out by a vet! :eek:
Re: the OP, my cats do it when there’s prey they can’t get to. If they can get to it, they’re deadly silent. So I interpret it as frustration.
Horses, lions, and other animals do things to create better air flow over the vomeronasal organ (Vomeronasal organ - Wikipedia). Flemen (or flehmen) is the term for this process of concentrating molecules to better detect odors. Many believe this to be what cats are doing when they “chatter”.
More reading:
To me, the OP’s vid cat sounds like he’s trying to imitate squirrel chatter. Some of my cats, while watching birds through a window, made an almost birdlike noise. Mine never made the OP’s cat’s noise.
The cat is Johnny LA’s link brought back scary memories for me. We had a cat that our vet said had brain lesions. The poor little guy would go into wild staring terror and yowling, just like the vid cat. Then he’d turn in circles and dash madly around the house. Then he’d stand in one place and pee on the carpet. The vet said we could give him daily anti-convulsive pills. However, the little guy already had a black belt in resisting pills. Sadly, we had to have him put down. He was not yet 3 years old. He was a beautiful, affectionate cat, and ne never got the chance at a good long life. Rest in peace, Nimbus.
This sounds the most plausible.
Diazepam is available as a suppository. Seriously.
lets see… can’t get cat to swallow pill, I know, I’ll try sticking it up his but!
(I know that’s probably easier, since its on the opposite end of the biting thing, but the thought process appears hilarious)
Anyone remember those photocopied cartoons that were passed around like spam back in the '80s? ‘Support the coffee fund. Put a dollar in the kitty.’ With the cartoon…
Some new data just in:
My neighbor had me watch her parakeets for a few days and they were placed where my cat could stare at them but not reach them. Obviously, he could smell them so if that’s the issue it explains why he didn’t do the chatter with them in the house. Also, he should have been frustrated with them in a cage and out of reach, so perhaps frustration doesn’t explain chattering. However, with the windows open and the outdoor birds flying near by, close enough and in such numbers that I would think he could smell them, he still does the chattering thing.
Discuss… : )
Weird. One of my cats does this, too, and it’s the only time I ever hear her chatter.
I’m pretty sure that’s what was being done to the cat in the video as well.
Two of my cats chatter at birds and squirrels out the window, and another, for reasons I can never explain, does it only when I cough in bed. He’s a weeeeird kitty. But I love him anyway. 
Interesting. I’ve always had a bunch of cats and now that I think about it, it is only the alpha that does this.
One of our cats who routinely chattered at bugs inside or birds outside would get seriously annoyed if we made that noise back at her. We figured it means something in Cat akin to “I’m gonna get you, you little &$^*(&@ and then I’m gonna tear you into little pieces…”
We call it the “ekk-ekk-ekk” and it is possible that it’s been used in the choruses of, um, meowed songs.
Apparently, you’ve not heard of Fred.