Whence is that knocking? –
How is ‘t with me, when every noise appals me?
– William Shakespeare
Macbeth, Act 2, Scene 2. A nervous Macbeth who has just murdered Duncan is terrified when he hears a mysterious knocking on the door. Macbeth is guilt-ridden at the enormity of the crime he has just committed and every noise alarms him. The person knocking is actually Macduff, who will eventually slay Macbeth.