Final answer:
Montresor heard a jingling of the bells just before placing the last stone, signifying the despairing movements of his captive, Fortunato.
Step-by-step explanation:
The only sound Montresor heard just before he placed the last stone was a jingling of the bells. This chilling moment occurs in Edgar Allan Poe's short story The Cask of Amontillado, where the protagonist Montresor walls up his enemy Fortunato in a niche within the catacombs. As Montresor seals the entrance with the final stone, the faint jingling sound signifies Fortunato's despairing movements, given that he is dressed in a jester costume with bells.