There is an interesting article on "Involuntary vocalization" by Harry Eyres at
http://www.guardian.co.uk/arts/fridayreview/story/0,12102,966166,00.html
The article mentions other pianists who hum and sing, but Glenn Gould is featured. There is even a link to an Audio clip of GG playing the Sinfonias "complete with humming". Try it out. Lots of humming, singing and talking.
"But was Gould's singing - and is the humming of pianists in general - an embarrassing encumbrance, even a sign of mental instability, or might it be inseparable from the expressiveness of their playing?"