This is Glenn Gould’s famous 1981 recording of Bach’s "Goldberg Variations." If you listen very carefully, you can hear him singing along. He sang so loudly that his recording engineers often couldn’t avoid it being picked up by the microphones.
Lots of musicians sing or hum along with what they are playing. Just to mention pianists, there’s also Bud Powell, Oscar Peterson and Cecil Taylor. You might call this pretense, or maybe a nervous tic, but I would disagree. Singing taps directly into the most musical parts of the brain and allows the music to filter through the body, rather than just the intellect. It’s a way to turn off the impulse to over-think or second-guess, because it’s impossible to be analytical and expressive at the same time.
Keith Jarrett is probably the most extreme example: he doesn’t just sing, it’s more like he performs a kind of gyrating dance at the keyboard - half the time he’s not even sitting on the bench. In his Köln concert of 1975, he makes faces, he hollers, he moans, he completely trusts his body.
Some have complained about all the extra noise, but to me it’s another, palpable way to hear a genius follow their muse.
Jarrett live in Tokyo, 1984: