What sound does the narrator hear at the end of the story that causes him to confess to the murder? What effect does the narration have on the story? Provide textual evidence to support your inference.

When the narrator hears the sound of the old man's beating heart he turns pale and nervous but the sound does not stop. It grows louder along with the narrator. He starts to talk louder too but the sound still increases. It states, "I foamed-I raved-I swore! 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." He also thinks that they heard it too and were mocking his horror. In the end, when the narrator can not stand it anymore he confesses to the police that he killed the old man and shows them where the body is. The narration has a huge effect on the story because it proves that the narrator in the end was going mad. He could only hear the sound. No one else.