My Ziggy does the chattering when she see's birds and I've also noticed her doing some lip smacking after she eats. She really enjoys her food so maybe she's just trying to savor the taste of her food.

Storm does what I'd call teeth grinding or chewing his cud. He'll sometimes just start making a chewing noise but he's not eating at the time. I've never understood why he does this. A friend of mines male cat does the same thing too.

Sorry to hijack your thread but I just wanted you to know that you aren't alone in the strange mouthing department.