While the narrator is hiding the body under the floorboards, he hears a knock on the door.
He opens the door to find two police officers wanting to know what that shriek was about and who it's from. He invites them inside.
What is that terrible sound?..... It's the old man's heart!
The narrator allows the police officers to check around the house, knowing that they will find nothing.
I ADMIT THE DEED! TEAR UP THOSE BOARDS!
The narrator then asks the police officers to sit down and have a chat with him. He puts his chair on top of the body with two chairs facing him.
As the narrator is talking with the cops, he starts to hear a faint heartbeat that steadily increases. The narrator thinks it's the old man's heart, and the mood is extremely suspenseful at this point.
As the sound of the heartbeat increases the narrator becomes anxious and eventually aggressive. He finally confesses the murder.