QUOTE(DEEP THOUGHTS)
It's quite possible at the same time Alan was either recording or mixing the "heartbeats" in one studio, another session in the building was recording or mixing that orchestral version of "Ticket To Ride" and through crossed wires, a shared patch bay or reverb chamber, wound up buried underneath what Alan was doing. (Orchestral/Muzak albums of Beatles tunes were so common back then.)
The Beatles, needless to say, did not record a great number of purely Orchestral versions of their songs

It's also suggested at the same place that it may have been from a radio broadcast...