Most likely what you are hearing is the 'chitter'. Some cats
'chitter' when they are hunting -- no one really knows why. It has
been speculated that they are 'calling' prey to themselves, not
unlike a duck hunter using a duck call. You will typically witness
the 'chitter' when the cat is extremely interested in something,
ears up, eyes dilated and focused, but the cat is not in a crouch
to pounce. Often it is seen in cats who are observing birds or
small rodents (squirrels, chipmunks, etc) out a window. The cat's
jaw will vibrate and the cat will make a 'ah ah ah ah' sort of
sound.