The narrator also hears immediately before he kills the old man. The heartbeat is significant of the narrator's guilt.
The narrator assures the reader at the beginning of the short story that he's not crazy, but more often than not, when Poe has a narrator insist that he's not crazy, the narrator usually is.
The heartbeat is other-worldly; the narrator is the only once who can hear it, and it makes him increasingly uncomfortable and drives him so insane so his only relief is to admit to the murder. It is either his own subconscious driving him to admit the murder or an other-worldly effect to drive him to the same conclusion--perhaps the old man's revenge from beyond the grave.
the narrator hears the heartbeat of the old man because he knows what he did was wrong and now he's guilty.
He can hear the heart beat because, he killed him and also because his buried in his house i think.