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.