My two don't chatter but Dudley trills and chirrups often, and Parker (9 & 1/2 months old) has just begun trilling to me. I find these sweet sounds delightful.
The difference between a trill and a chirrup is a trill is done from cat to human, whereas a chirrup is done from cat to cat. Both sounds are an expression of joy or anticipation of joy (as Dudley chirrups in the hall in order to get Parker to go and join him for a game of 'ninja pounce and roll').
My Maine Coon mix would get the chattery jaw and noise when she was watching birds - the Big Guy doesn't bother with that LOL but he sure has some darling trills and purry meows
My little girl makes little noises every time she moves. We've had her checked out, and the vet says she is fine. Weve come to the conclusion thay its her way of letting us know she is there.