don't worry. the odds of something like that happening again, especially in New York, are low. you are probably worrying yourself which is causing the dreams. if you were personally effected by these events, and as you said noticing 7 year cycles, your suppressed fears could be flaring up causing the dreams. talk to someone about these feelings if they are suppressed emotions surrounding the attacks.
it could also be symbolic. without knowing the full dream, it could be warning of danger surrounding a trip, or if you were with your partner in the dream it could warn disaster with them. not saying a planned outing or relationship will result in death, but it could end badly. a trip could be full of stress, or you loose your luggage, or break your leg. a relationship could become abusive or they may cheat. this is if this dream is a warning. look at the whole.