The Clue of the Tapping Heels (Nancy Drew #16)

Challenging questions confort Nancy Drew when she attempts to solve the mystery of the strange tapping sounds in the house of a retired actress. Who is the tapper? How does he gain access to Miss Carter's house, despite securely locked doors and windows? Why do the tapping sounds come in Morse code? Is there a sinister motive behind the prowler's actions"?