I'd check all of the above, but if those all check out, consider that it could also be just the tire itself. Not only do these sport tires wear a lot faster, but their performance diminishes dramatically as they wear, and as they wear they have a tendency to hum.
All tires have harmonic resonance when they roll, and that resonance will change under different load conditions and as the tire wears. I've noticed particularly on the Mazda 5 that certain tires will hum more than others, and they get louder as they wear. This is due in part to the wide tread and low profile, and tends to be more noticeable on the rear due to the vehicle's weight imbalance. The front tires wear faster due to the work and the weight... So you take those more worn tires, and stick them on the back, where the load is much lighter. Then you notice the hum much more because it didn't develop over time with normal rear tire wear.
A friend of mine had a Mazda 5 that did this. I swore it was the bearings, so I had the dealer swap her tire/wheels with mine (which had brand new tires at the time)... now all of a sudden her car sounded fine and mine hummed like a humming bird. Eventually she got new tires and the hum went away.
