The answer to this is a little tricky. “The Tell-Tale Heart” is a short story by Edgar Allan Poe, and he’s well known for his unreliable narrators. He was also an American writer, so if the answer were really “mad”, it would have to be in the British sense of the word: crazy. The story centers on a man that has murdered his friend, and buried the body under the floorboards of his house.
The police come to find the friend, and all this man can hear is the “thump thump” of his friend’s heart. Since the heart died with the body, it cannot thump any more. Therefore, the man is trying to convince the audience, and the police, that he’s not crazy.