Perhaps it's a story about siblings. The older sibling scolds the younger one playfully when the younger one tries to steal a cookie from the cookie jar. The 'ah ah ah' could be the older sibling's initial reaction, a sort of vocal prelude to the scolding. The fanfic could then explore their relationship further, like how the younger sibling reacts and how they make up afterwards.
In a fantasy world, a wizard scolds his apprentice playfully. The apprentice has been misusing a simple spell for fun, like making small animals talk in silly voices. The wizard's scolding is full of affection, and the 'ah ah ah' is his unique way of expressing his amusement and mild disapproval at the same time. This could lead to a series of adventures where the apprentice learns to use magic more responsibly.
One possible plot could be that in a school setting, a strict but kind teacher scolds a student playfully. The student might have done something minor like showing up late with an excuse that was clearly made - up. The teacher's scolding is more of a light - hearted reprimand, with the 'ah ah ah' being the teacher's way of feigning exasperation, and the student taking it in good humor.