The following FAQ entries may contain spoilers. Only the biggest ones (if any) will be covered with spoiler tags. It is assumed that no one who is diligently avoiding spoilers will be visiting this page in the first place.
For detailed information about the amounts and types of (a) sex and nudity, (b) violence and gore, (c) profanity, (d) alcohol, drugs, and smoking, and (e) frightening and intense scenes in this movie, consult the IMDb Parents Guide for this movie. The Parents Guide for Taxi Driver can be found at http://www.imdb.com/title/tt0075314/parentalguide.
Scorsese himself has commented that it was meant to signal to the audience that Travis could have a fit of rage again at any point in the future and that he is not the glorious hero the newspapers make him out to be. The sound itself was originally a cymbal clash. However, Scorsese felt that it sounded too cliché so Bernard Herrmann had the idea to play the sound backwards, making it sound unusual and threatening.
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