The police was called to search the home, and the narrator was confident with nothing to be afraid of. They let the police in the room of the murder crime and spoke to them and answered their questions guilt-free, to show his confidence. In the midst of speaking with the police, he began to hear a heartbeat from the old man. That being impossible, due to the fact that he cut the old man’s body in pieces. We now are aware that the real heartbeat was coming from the narrator himself. The guilt was building up inside of him, making nervous and scared, to the point where he broke down and confessed/revealed his action.
That is where the title comes from; he was revealing/confessing something who could not keep to himself, which was the murder. He was confident at first, but the guilt builds up and takes over his body and causes him to revealing the truth to the police officers. Now, if we go back to the first sentence, when he was denying the fact that he was crazy because after reading his story on what had happened, the average person would assume, or even know, that this unknown narrator was