In "The Tell-Tale Heart," what does the narrator hear?

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From the very beginning of this rather frightening Gothic classic of Edgar Allen Poe, the narrator who tells us his tale places emphasis on the senses and how his "disease" which goes unspecified, had actually "sharpened" his senses. Thus it is that in the first paragraph he makes a rather bold claim that perhaps is the beginning of our suspicions that he may not be entirely reliable:

Above all was the sense of hearing acute. I heard all things in the heaven and inthe earth. I heard many things in hell.

Throughout the story this acute sense of hearing is returned to, especially as he hears the old man's heart, both before and after his death. This acute sense of hearing clearly comes back to plague him as in the last paragraph, whilst he is talking to the officers investigating the sound of the dead man, he hears once more the sound of his heart:

Yet the sound increased--and what could I do? It was a low, dull, quick sound--much such a sound as a watch makes when enveloped in cotton.

The ambiguity of this story is created by the fact that the officers never hear this sound. It is only the narrator with his acute hearing, leaving us to ponder whether this sound emerges from heaven--or from hell, as the narrator tells us he can hear sounds from both locations. Either way, it causes the narrator to confess his crime.

According to the narrator,

... I heard a slight groan, and I knew it was the groan of mortal terror. It was not a groan of pain or of grief—oh, no!—it was the low stifled sound that arises from the bottom of the soul when overcharged with awe.

I believe it was just this: the old man's groan upon realizing that there was something--or someone else--in the room, something that would eventually lead to his death. For an hour both of the men remained motionless and virtually silent: the old man waiting for something to appear, and the narrator--the killer--awaiting the perfect time to make his move. Unlike the other sounds the narrator imagines, whether it is the beating of the old man's heart or his own; or whether it is something else, like the "death watches" ("a type of small beetle that lives in wood and make a ticking sound"--like a watch); this time it is not imagined but the real breathing and the terrified sounds that come from the old man as he awaits the terrible consequences that are soon to come.

In "The Tell-Tale Heart," whose heartbeat does the narrator hear after the old man's murder?

This is an excellent question to consider. It is very important to be aware that there are a number of different alternatives when we think about how we might answer this question. The heartbeat that the narrator hears at the end of the story as he is being questioned by the police is a sound that he definitely feels comes from the location where he has hidden the body:

It was a low, dull, quick sound--much such a sound as a watch makes when enveloped in cotton.

However, what is key to realise about this story is that Poe deliberately creates a narrator who is unreliable. His grandiose claims towards being able to hear everything at the beginning of the story and his clear madness makes us suspect his account. One sensible conclusion that we therefore might come to would be that this heartbeat is actually his own imagination. We could argue that given the way that the sound starts when the police arrive, that it is actually a product of the man's own conscience, his beating heart of remorse that will not be satisfied until he has confessed to the crime that he has so callously committed.