Vibrato. It’s the same in vocals, changing pitch quickly in a way that sort of sounds like a vibration, though the technique itself is not called vibration.
After Lennon's death the idea of reunion practically vanished.
Still, Paul McCartney, Star and Harrison got together during their work on "Free as a Bird" (which included the vocals of the late Lennon) for "Anthology 1", released in 1995, and for "Real Love" for "Anthology" 2 ”from 1996.
