Why does the sound of the old man's groan in the middle of the night sound familiar to the narrator of The Tell-Tale Heart?

The sound of the old man's groan is familiar to the narrator because it triggers a deep sense of guilt within him. The groan reminds the narrator of his own internal turmoil and the fear of being discovered for the murder he committed. Ultimately, it symbolizes the narrator's increasing paranoia and descent into madness.