It is also very possible that the heard the death watch beetles in the walls, which Poe mentions in the course of the story:
He was still sitting up in the bed listening;—just as I have done, night after night, hearkening to the death watches in the wall.
This type of beetle makes a thumping sound, very similar to a heart beat. The policemen might not have even noticed the sound or given it a second thought, but the narrator, being wracked with guilt and paranoia, hears the noice and assumes the worst.
I have to agree with the others; only he hears the beating of the heart. That said, I have often interpreted it as his own heart beating (given his guilt). Although I have thought about it being an audial hallucination, I have always decided that it is his own heart beating in his ears.
He could be hallucinating because he was crazy to begin with. Sane people don't normally kill because they think someone has an evil eye. After he killed the old man, he was desperate and wracked with guilt.
Yes, at the end of the Edgar Allan Poe story, The Tell-Tale Heart, only the narrator hears the beating of the old man's heart because of his guilt. No one else can hear it because the man is dead and the heart is not beating. The narrator, who has said all along that he is not crazy, reveals his craziness here when he reveals that he can hear the old man's heart telling on him with its beating and that he is the killer or the guilty one. The policemen are preparing to leave, convinced by the narrator that all is well in the household.