Humming on rare occasion isn't a problem; humming all the way through a
work is another matter.  It amounts to a re-instrumentation of the work
performed.  An instrumental work becomes a kind of vocal work.  If Gould
couldn't help himself, as you suspect, neither can we, in finding his vocal
noises an intrusion on a work that requires a respectful silence as much
from the musician performing the work as it does from audience members
listening to it.  How would you feel if someone sitting next to or near you
at a concert were to hum along audibly with the music? I suspect it would
drive you nuts.  I also don't think that making a racket of any kind while
performing a work is justified by the performer having been judged
outstanding at his profession, by "some" people.  Often in listening to
recordings, I'll hear various vocal sounds from conductors.  Sometimes it's
insignificant, at other times it ruins a passage here and there.  Perhaps
some of these conductors would be less likely to make these noises if they
were reminded that these sounds would be heard by posterity in recordings.

John P.