knocker

n.
  1. One who, or that which, knocks; specifically, an instrument, or kind of hammer, fastened to a door, to be used in seeking for admittance.
  2. A person strikingly handsome, beautiful, or fine; one who wins admiration; a “stunner.”
  3. A species of large cockroach, especially Blabera gigantea, of semitropical America, which is able to produce a loud knocking sound.