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 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...