Believe it or not, Alfred Brendel hums in his recordings.
This is surprising. I don't even know if Brendull's mind is even switched on when he records.
Listen to his performance of the Arietta of Beethoven's sonata no. 32. 6:44 into the piece, at the beat before the entrance of the majestic "boogie-woogie/jazz variation," he lets out a groan. So apparently he's actually reacting to the music, which is very surprising and unusual for him.