Question

In "The tell tale- heart"by Edgar Allan Poe ,what does the beating heart that the narrator heard symbolize

Answer 1

In "Tell Tale Heart" by Edgar Allan Poe, the beating heart the narrator heard symbolizes, at least in my opinion, the narrator's guilt. 
He killed a man, and he kept hearing his beating heart, even though it was impossible. He kept hearing it because he felt guilty because he murdered a person, and he had to come clean to the police officers in some way. If he hadn't, he would go completely mad, and his crime would go unpunished, probably. 

Answer 2

I'm not sure the word for it, but I'd say it symbolizes the truth to a game the narrator was trying to play. When he was trying to lie and make sure that no one knew about the body under the floor, he almost succeeded in doing so, until he thought he heard the man's hear beating from under the floor. Hearing the sound of a beating heart in his ears made him go insane until he finally just confessed about the body under the floor. 
I hope my answer kind of helped you.