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.
Even though synonyms generally share the same meaning, they might not apply in the same context. So, if you want to use one specific word you should do that instead of trying to find a synonym that doesn't quite fit into the context you've established. An example is if you want to use the sentence "I was mad", meaning "I was angry", you could look for a synonym. One synonym is "absurd". However, this is a different type of mad, meaning crazy instead of meaning angry. "I was absurd" has an entirely different meaning than originally intended. This is why you should always double check the contextual meaning of the synonym you want to use.
