In "The Tell-Tale Heart," what effect does the visit from the police have on the narrator?

When the police first arrive at the narrator's apartment, he is feeling supremely confident; he is so confident, in fact, that he brings in some chairs for the officers and places them directly over the spot where he's buried the old man's body.  He says, "I was singularly at ease."  However, the longer the officers sit there, the more nervous the narrator gets.  He hears a ringing in his ears that increases in volume until he begins to think that it is not inside his own head but outside.  He talks louder to cover it up.  He describes it as "a low, dull, quick sound -- much such a sound as a watch makes when enveloped in cotton," and this is precisely the way he described the sound he thought was the old man's heartbeat, just before the narrator killed him.  However, the old man is dead, and so the sound must be the narrator's own heartbeat, speeding up from his adrenaline.  The narrator grows suspicious that the officers suspect him of murder, and he eventually confesses.  Although he is calm at first, the presence of the police makes him so nervous that he guiltily confesses, having misinterpreted the sound of his own rapid heartbeat as well as the behavior of the officers who "chatted pleasantly, and smiled."