That sound is usually the sound of something not being right (i.e. danger). My chickens do it anytime they see local wildlife that they haven't seen before, might eat them, or HAS eaten them before (I have a fox problem). They also do it when a new adult comes into their coop (but not kids). And they alert each other when a cat walks nearby, but then realize how small my cats are compared to themselves, and don't start the other noise.
And of course, my bantams do it when they are laying an egg, probably because it hurts.
So if your chicken is trying it, see if there may be an animal (or person) walking near them when the chickens are out. If she does it when only YOU are around, work on getting her to trust you with treats.