Hello,
Tires have a tendancy to hum on the right type of surface. Try driving on different roads...Like Concrete, vs Pavement.
Only other thing I could think of, is.....Is your steering wheel straight when going down a straighaway? If not, you may be in serious need of an alignment.
I don't think wheel bearings hum, allthough I may be wrong there. I always thought they started knocking when under stress...not humming at given speeds.
One more thing, make sure all your body panels are secure. Also the front air dam under the fog lights. Might be the wind you're hearing whistling through a loose connection.
I got nothing else. Good Luck!
Later now,