This is "The Tell-Tale Heart" By Edgar Allen Poe:

True! — nervous — very, very dreadfully nervous I had been and am; but why will you say that I am mad? The disease had sharpened my senses — not destroyed — not dulled them. Above all was the sense of hearing acute. I heard all things in the heaven and in the earth. I heard many things in hell. How, then, am I mad? Hearken! and observe how healthily — how calmly I can tell you the whole story... It all began one morning when I was in the middle of a demon-summoning ritual when I received a strange knock on my door. I ignored it, as I needed to focus, but then the knock grew more and more persistent, getting to the point where I just couldn't take it anymore. I grabbed my shotguns, loaded and cocked it, and headed for the door, ready to kill whoever was interrupting me. When I opened the door, I saw an odd man, standing on my porch with all black on, a black cloak, and a black hood, concealing his true identity from me, it is possible that he had a rather unpleasant face, or that he had ill-intent for me. In his arms, he cradled a wooden box. "Salutations, sir.", he said in a deep, somewhat sinister voice. "I would like to inquire as to if you would be interested in this.". He handed me the box, I was flummoxed as to what the devil it was. I reached out and looked at the box. This is what he interrupted me for? "This is not just any box.", Said the man. "This box contains the tell-tale heart." I gazed at him, perplexed.

"The tell-tale heart?"

"Yes, it is the heart of Agathodaemon, the god of fortune. Whenever something bad is coming, it beats. Great way to prepare yourself for what's to come."

I glared at him, still puzzled about what he was talking about. For you see, I do not believe in this mumbo-jumbo, supernatural stuff. I only believe true facts that are grounded in the real world. "