I have the stock config'd Mac Pro (so no noisy ATI card, sadly), and I'd say the humming of the computer is very slight. It resides in a fairly quiet room, and while it's audiable, it isn't distractingly so.
Now, the HD in there, on the other hand, is a heavy clicking machine. Spotlight indexing makes the HD sound like machine gun fire in the far distance. I'm not quite happy with that, but it's only all that noticeable because as a whole the machine is so quiet. My Superdrive is blazingly loud though... (the Pioneer, not the "Sony"), much louder than anything else. Even when I updated the firmware, and the fans were on full, they didn't compare to the explosion of sound that comes from my Superdrive. Annoying, but not enough to make me unhappy with my purchase.