I'm reading a book by Lilian Jackson Braun and she writes "KoKo said "ik ik ik"?

Question

I've never heard a cat make this kind of noise so what does she mean?

Answer 1

Cats will make that noise when they have spotted something (usually a bird or mouse) that excites their curiosity... their mouths will literally click up and down very fast and that peculiar little noise will be heard.

Answer 2

Koko as I recall is Siamese, and this is probably the "chattering" noise made while looking out a window at a bird.