What is the clincher for entire selection of Tell Tale Heart?

The narrator says that he carefully removed the bed's planking, took out the old man's body, severed his head, and concealed it all beneath the planks before re-nailing the bed with unusual precision. The detectives arrived hours later, and the self-assurance of the narrator allows him to remain composed throughout their inquiry, even when they search his house and the basement, confident in the fact that they will find nothing incriminating. The narrator, however, becomes aware that a noise that keeps growing louder is ringing in his ears and is getting louder. He starts to think he's going crazy because it sounds exactly like a heartbeat. In desperation, he eventually cries out, "I admit it! Tear up the planks! Here, here! It is the beating of his hideous heart!" With this confession, the story comes to a chilling conclusion.