April 21st, 1967 Working at Abbey Road studios in London, The Beatles completed the sessions for Sgt. Pepper’s Lonely Hearts Club Band.

The final recordings were a short section of gibberish and noise which would follow ‘A Day in the Life’, in the run-out groove. They recorded assorted noises and voices, which engineer Geoff Emerick then cut-up and randomly re-assembled and edits backwards. At John Lennon’s suggestion, they also added a high-pitch 15 kilocycle whistle audible only by dogs. These were omitted from the American version of the album.

Source: On This Day in Music