"There came forth in return only a jingling of bells. My heart grew sick- on account of the dampness of the catacombs."
It seems that Montresor feels guilty for what he's done to Fortunato, burying him alive among the tombs of the estate. He does not want to feel guilty, but rather he wants to feel satisfied that his revenge has been exacted so he brushes off the heartsick feeling as coming from the dampness. He thought that his revenge would feel much sweeter than what it is he is experiencing and would not dare admit that what he's done is wrong and far too harsh a punishment for any unnamed insult Fortunato may or may not have committed against him. What is worse, Fortunato will die not knowing what it was that he did to his so-called friend.