There it is again

Boy: Oh, oh, it's getting windy outside. I don't like it when

it's windy like this. I... I feel scared. Oh, what's that knocking sound?

.

.

Boy: Oh, why did you knock on the door just now?
I mean it's your workshop. Surely you don't have to knock.
Mom: I didn't knock.
Boy: Then who did? There it is again.
Mom: It's the window.
Boy: Someone's knocking at the window?
Not someone, something.
Boy: Oh, maybe the cats and dogs are trying to get in. Ha, ha.
Mom: Ha, ha. No, it's not the cats and dogs. It's the branches

of the tree outside.

Boy: Oh, why are the branches knocking on the window?

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What does "There it is again" mean?

Thank you