A noisy elevated train used to run along Third Avenue in New York City. After it was torn down, "many people in the neighborhood began to call the police quite late to report something strange occurring "unusual noises, suspected thieves or burglars. . . . The police determined that these calls took place at about the time the former late-night train would have passed these people's houses. What they were hearing' was the absence of the familiar noise of the train." 

They had grown habituated to this particular noise. Now they would have to become habituated to its absence. Habituation. Growing so accustomed to something that we no longer even realize it is there.