It's right after the narrator kills the old man so he tears up the floor boards and hides the dismembered body under there.
I then took up three planks from the flooring of the chamber, and deposited all between the scantlings. I then replaced the boards so cleverly, so cunningly, that no human eye --not even his --could have detected any thing wrong.
The narrator hears a knocking at the door and it's the police! They told him that a neighbor heard strange noise. Since the narrator hid the body so well he let them come in and look around.
I smiled, --for what had I to fear? I bade the gentlemen welcome. The shriek, I said, was my own in a dream.
It's the final scene and the narrator thinks he can still hear the old man's heart beating so he confesses.
"Villains!" I shrieked, "dissemble no more! I admit the deed! --tear up the planks! here, here! --It is the beating of his hideous heart!"