I hum when I play sometimes, it just come naturally. Humming is positivly fun.
Yes, but positively fun for whom? Maybe for the pianist.
Speaking as audience, I find humming pianists very annoying.
Yet, you would be surprised at how many of them – besides Glenn Gould - do (or did) it: Rudolf Serkin, Andre Watts, Grigory Sokolov Maurizio Pollini, Stephen Kovacevitch, Alfred Brendel and Richard Goode (who does not really hum, he sings really loud!) to name just a few.
Check this out:
https://www.guardian.co.uk/arts/fridayreview/story/0,12102,966166,00.htmlhttps://glenngould.org/f_minor/msg08423.htmlApparently humming gets worse with age; Brendel, Pollini and Watts were not known for their humming in their early years.
I agree with Saturn: These guys should just discipline themselves not to do it.
By the way, here is a thought: Is it gender based? I cannot recall right now any woman pianist who hums! Is that the ultimate gender difference between pianists?

However even more infuriating than pianists humming is when the audience starts humming! The other day, I was showing a student what Mozart’s Turkish March (from sonata K331) sounded like, and her aunt - which was listening to the lesson half asleep – wake up with a jolt and started humming (tralalalaing would be more accurate), and clapping! For crying out loud! What is it with these people?

Best wishes,
Bernhard.