Onomatopoeia is a literary device in which a word is formed to imitate a sound. Examples include "ring," "pow," "snap," and "boom."
The first example of onomatopoeia in Edgar Allan Poe's short story "The Tell-Tale Heart" is the word "creaked." It describes the sound the lantern made when the narrator lifted the slats to reveal the light.
And then, when my head was well in the room, I undid the lantern cautiously—oh, so cautiously—cautiously (for the hinges creaked)—I undid it just so much that a single thin ray fell upon the vulture eye.
The next example is the word "groan." The old man wakes up startled when the narrator slips and makes a noise with the lantern. After asking who is there and hearing nothing, the narrator describes his groans of terror. The word groan describes the sound that came out of the old man's throat. The quote is below:
Presently 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 knew the sound well.
The third example is the word "chirp." In the story, it is describing the sound a cricket makes by rapidly rubbing its legs together. The narrator is speculating as to what the old man is thinking. He supposes he is rationalizing the noise by saying it was the wind, or a mouse, or a cricket:
He had been saying to himself—"It is nothing but the wind in the chimney—it is only a mouse crossing the floor," or "It is merely a cricket which has made a single chirp."
The next example occurs when the police come over to investigate after the neighbors report the shriek they heard. He is chatting casually with them, and then he becomes uncomfortable and wants them gone. The narrator states: "My head ached, and I fancied a ringing in my ears." The word ringing describes the sound in his ears and is another example of onomatopoeia.
The sound of the heartbeat itself is not described with onomatopoeia because it isn't really happening. It is all a figment of the narrator's imagination.