2005 Mazda 3 vibration

2005 Mazda 3 GT (5spd), 40,000 km, 2wd.

Just had new tires put on, rotated and balanced etc. Now, there is NO vibration until I hit about 80 km/h, then all of a sudden there is a detectable vibration I can feel in the car, but the steering wheel does not shake. Vibration is less as I approach highway speed from 80 km/h up to 110 km/h. Took the car in, they said the balance was fine and there was no separation on the new tires. What could be causing this vibration in such a new car? The tires may be a red herring, I'm not sure if that is indeed the source of the problem or just coincidence.