At the end of the scene, Macduff and his companions are arriving at the castle. They apparently had been told by Duncan to come early-- indeed, earlier than Macbeth had expected them. The last line of the scene has Macbeth shouting, "Wake Duncan with thy knocking! I would thou could'st." The exasperated plea is similar to the cliche that something is so loud, it could wake the dead. Macbeth immediately follows up with a statement of guilt. He regrets what he has done, and the guilt will subsequently consume him.