Are you sure it was an electrical hum and not just hot items (coolant, oil, metals, etc.) cooling? Also, it's "possible" that air temperature differentials (and pressure) from one side of a fan to the other will cause a fan to rotate, but I wouldn't think it would spin fast enough to hum. My final thought is that the fan would continue to spin from inertia for a brief time with no power. Other than those, I've got nothing...but it's always fun to just join a discussion.
