In the story "The Black Cat" by Edger Allen Poe, what happened when the narrator tapped on the brick?

Toward the end of the story "The Black Cat" the narrator does more than just tap on the brick. He describes his actions as "rapping heavily" upon the brick wall with a cane. 

At this point, the narrator has clearly lost his mind. He is ridden by guilt, anger, the toxicity of alcohol, his own personal demons, and the fear of being found out. He is feeling all of this at the same time. 

Yet,...

Toward the end of the story "The Black Cat" the narrator does more than just tap on the brick. He describes his actions as "rapping heavily" upon the brick wall with a cane. 

At this point, the narrator has clearly lost his mind. He is ridden by guilt, anger, the toxicity of alcohol, his own personal demons, and the fear of being found out. He is feeling all of this at the same time. 

Yet, as the police comes to visit and check the premises "four days after the assassination", the cellar is checked and all clears out, but the main character is still in a daze of insanity not knowing exactly what he is saying to them: 

"Gentlemen," [...]"I delight to have allayed your suspicions. [...] these walls are solidly put together;" and here, through the mere frenzy of bravado, I rapped heavily, with a cane which I held in my hand, upon that very portion of the brick-work behind which stood the corpse of the wife of my bosom.

Basically, what takes place here is that the narrator gets arrogant at seeing that he may have escaped the consequences of justice. He starts then making a big deal out of his cellar being well-built, he taps on the bricks, and what happens next is that he hears a voice coming from within the makeshift tomb. He describes it as a child's cry that is muffled, and then changes into a long scream, a howl, that is as wild as it is inhuman. 

When the policemen hear the sound coming from the wall, they all break through it and find the corpse of the woman with a cat right atop her head. This cat's cry was what gave away the secret, and it was the reason why the narrator will be executed for murder.