"I Feel Fine" starts with a single, percussive feedback note produced by McCartney plucking the A string on his bass, and Lennon's guitar, which was leaning against McCartney's bass amp, picking up feedback. This was the first use of feedback on a rock record. According to McCartney, "John had a semi-acoustic Gibson guitar. It had a pickup on it so it could be amplified ... We were just about to walk away to listen to a take when John leaned his guitar against the amp. I can still see him doing it … it went, 'Nnnnnnwahhhhh!' And we went, 'What's that? Voodoo!' 'No, it's feedback.' 'Wow, it's a great sound!' George Martin was there so we said, 'Can we have that on the record?' 'Well, I suppose we could, we could edit it on the front.' It was a found object, an accident caused by leaning the guitar against the amp." Although it sounded very much like an electric guitar, Lennon actually played the riff on an acoustic-electric guitar (a Gibson model J-160E), employing the guitar's onboard pickup.

Later, Lennon was very proud of this sonic experimentation. In one of his last interviews, he said: "I defy anybody to find a record – unless it's some old blues record in 1922 – that uses feedback that way."