could also be a wheel bearing in need of replacement. Usually they hum around 40 mph but you can try lifting the car, have someone hold speed (make your e brake is on, blocks behind it, make sure its on GOOD jack stands...........) and put either a stethoscope or a screwdriver to rear of the bearing if nothing is in the way and see if you can hear excessive noise.

Bearings will also make bad "barking" or "loud rubbing" sounds when backing up slow or driving foward very slow. Most of the time they are pressed and sealed meaning they aren't serviceable. So you can't really take them apart to check the bearing case. Almost every case you will need to replace the bearing hub with another.