What was the narrator really hearing at the end of the story the tell tale heart?

At the end of "The Tell-Tale Heart," the narrator believes he is hearing the sound of the old man's heart still beating beneath the floorboards where he has hidden the body. This auditory hallucination symbolizes his overwhelming guilt and paranoia, ultimately leading him to confess to the murder. The incessant sound reflects the psychological torment he experiences as he cannot escape the consequences of his actions. Thus, what he perceives as a heartbeat is actually a manifestation of his own conscience.