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? for the story tell-tale

"The Tell-Tale Heart" by Edgar Allen Poe is a brilliant story with the theme of "even if you don't tell anyone when you commit a crime, your guilty mind will tear you apart". Near the end of the story, the narrator begins hearing the sound of the dead man's heart beating. This causes the narrator to go crazy enough to confess to the murder to the cops. The narration is very interesting. The story begins with the narrator claiming that he is not crazy. This immediately causes the readers to feel unsettled. Over the course of the story, as the narrator accounts his completely unjustified hatred for the old man with the strange eye, the readers come to realize that the narrator is crazy.  

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