Possible Clues:

• Short sharp noises of a knock on the door
• Sharp knock as on door
• Sound of a door knocker
• Sharp sound of a door-knocker
• A sharp knock on the door
• Noise made by rapper with short cane at end of act
• What a blow that on the door there's pestiferous rubbish
• Drumroll sound
• Sharp knock at the door of opium dens outside studio apartment
• Knocking syllables