This isn't Boots, but he does this at the laser dot.
Maybe it's a tuxedo cat thing?
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!