However, I also found this: Who's right?Marianne Faithfull, Anita Pallenberg, Brian Jones, Charlie Watts, producer Jimmy Miller, Wyman and Richards can be seen performing backup vocals in the film Sympathy for the Devil by Jean-Luc Godard, but this scene was staged specially for the cameras; the actual backing vocals were overdubbed by Jagger, Richards and Miller.
http://www.last.fm/music/The+Rolling+Stones/_/Sympathy+For+T...The "Whoo-Whoo" backing vocals were added when Richard's girlfriend, Anita Pallenberg, did it during a take and the Stones liked how it sounded. Pallenberg sang this on the record along with Keith Richards, Brian Jones, Bill Wyman, Marianne Faithfull and Jimmy Miller. (thanks, Rich - Midland Park, NJ)
http://www.songfacts.com/detail.php?id=509