I've held off buying parts to address this whining/humming noise because I'm unsure of what's actually causing it.
Well, its got worse and it's loudest between 40 and 60 mph and doesn't respond to acceleration or deceleration so I'm leaning towards it being a wheel bearing, and I'm not sure if its front or rear.
I'm thinking of getting four of the SKF BR931101 for around $100 each on RockAuto in the hope that will cure the irritating noise - or wait until it gets a lot worse making it more evident.