He sang through a Leslie speaker.

That warble you hear isn’t an instrument—it’s his voice run through the rotating speaker from a Hammond organ. Lennon wanted to sound like the Dalai Lama chanting from a mountaintop, so they fed his vocal through a Leslie cabinet. He also messed with tape loops, but the voice-through-Leslie trick is the weirdest and most iconic.

It’s not a secret or a trivia trap. It’s just a creative studio hack that changed psychedelic music.