Yup, my cats chatter.. mostly at the birds, squirrels, and chipmunks outside though. I've never really heard it during play. Its absolutely adorable though.
Mine 'chirp' or 'chatter' at animals and birds outside. It is a normal excited reaction at seeking possible prey. It is very funny watch them do it, though, especially accompanied by tail waving and body trembling. It is as if they are uncontrollably excited.
Scarlett chatters at leaves that blow by the sliding doors and windows. Fall is her favorite time of year. If I open up a sliding door when she sees a leaf, she'll run out, grab the leaf and run back inside. She loves to play with them.
Muddy has an entirely different type of chatter. It's loud and we call it his Harley Davidson noise. He does it at food times, and in fact, he's sitting at my feet right now reminding me with his chatter that it's time for his breakfast. I guess I'm off to feed the little piggy now!
My cat, the one who looks just like Boots, does the same thing. His meow is different, so he doesn't make that exact same noise, but it's definitely a chatter.