Answer:
The sound that triggers Macbeth's fear in Act 1, Scene 2 is the sound of knocking at the gate to the castle. Macbeth has just killed Duncan, and when he hears a noise, he immediately assumes that someone is coming to make him pay for what he did. Macbeth realizes that although he will become a king, the rest of his life will be filled with fear of death. In this scene, Macbeth is also wondering whether the ocean will wash the blood from his hands, and comes to a conclusion that he will actually never be free of guilt.
