Tri-Star Pictures intends to appeal the X rating given to Alan Parker's "Angel Heart" by the ratings board of the Motion Picture Assn. of America. The occult thriller based on William Hjortsberg's sex-filled and violent novel "Falling Angel," stars Robert De Niro and Mickey Rourke as a client and a private eye who follow the trail of a missing big-band singer into 1950s New Orleans. A ratings board spokesman declined to say Monday why "Angel Heart" was given an X, which is regarded as the commercial kiss of death by film distributors, and said that any appeal would be heard by a board in New York. "Angel Heart," which co-stars Lisa Bonet from the hit TV series "The Cosby Show," is scheduled for national release March 6.