When the narrator tapped on the wall, the cat began to cry out, for it had been trapped inside the whole time. This eventually led to the arrest of the narrator for the murder of his wife.

Explanation:

In Edgar Allen Poe's short story "The Black Cat', an unknown unnamed narrator tells of how he ended up being in jail waiting execution.

In the story, the narrator tells us that he used to have a favorite cat named Pluto who used to be his constant companion. Even when he's angry with everyone, the cat never leaves his side. But on one instance of anger in his drunken state, he removed one eye of the cat. Feeling that he is completely ignored by Pluto now, he sought revenge and under the influence of alcohol, he hung the cat from a tree. With this, their house mysteriously caught fire and burned the whole house, except a wall with an imprint of an image of an outline of a massive cat hanging from a tree.

Later, he acquired another cat which strangely resembles Pluto with one eye. But as like before, he began to resent this cat too. On one occasion, he almost tripped over the cat. Enraged, he tried to hit it but was defended by his wife. In anger for her defending of the cat, he killed her. Then he tried to conceal his crime by putting the corpse on a wall and building another layer on it. The cat was also locked in with the body, though unintentional. Later, when the police came to investigate the mysterious disappearance of his wife, he showed them the house. Just before they left, in his eagerness to show that he is innocent, he tapped on the wall to show off, but the imprisoned cat gave a loud cry. This made the policemen take down the wall to check it. Finally, the corpse was discovered and thus, that's how he ends up in prison.