The narrator confessed to his crime because he kept hearing the heartbeat getting louder. As the narrator was talking to the officers he heard the sound that he had heard before murdering the old man. The narrator got paranoid for only he could hear the heart beating and the officers had not idea what he had done and he thought that if he could hear the sound, then the officers could too, but they couldn’t but he did not know that so he thought the officers were taunting him, “ I swung the chair upon which I had been sitting, and grated it upon the boards, but the noise arose over all and continually increased. It grew louder -- louder -- louder! And still the men chatted pleasantly, and smiled” ( Poe 6). If he didn't feel guilty about murdering the old man then he would just join the conversation and ignored the heart but, the narrator, instead, noticed and the noticed that the officers did not hear so he ignored it but then it got louder. The narrator would not even have noticed the heart if he didn't feel guilty He was the only person in the room who could hear it because his paranoia was getting into his conscience and making him feel the guilt he tried to cover but failed