Hi! This is what he is hearing in this novel:
So, Edgar Allen Poe describes his character as a maniac, we know that. After the murder of the old man, the main character describes a thumping sound that keeps reappearing. This is the sound of the heart thumping! We also know that the heart of the old man is under the floor of the murderer's house!
How terrifying!
Anyway, sometimes when a person is mentally challenged (like the murderer) they often tend to hear or see things that make them go.... crazy! Basically this man was going insane from the guilt of what he had done.