Final answer:
The sound of the old man's heart symbolizes the guilt of the narrator.
Step-by-step explanation:
The sound of the old man's heart in the story most closely symbolizes the guilt of the narrator. The beating of the heart represents the narrator's overwhelming guilt for committing a horrible act, which he cannot escape. It serves as a constant reminder of his wrongdoing, haunting him throughout the story.
Learn more about Symbolism in literature