Tires can make the same kind of noise a wheel bearing does. Did you check them ? Check for uneven wear and run your hand over the surface. The tire should feel smooth with no bumps or sharp edges on the threads.
Did you try jacking up the car and run it while off the ground ? That will sometimes help pinpoint the location. Have someone else at the steering wheel while you listen. Be careful though. A spinning wheel isn´t something you want to get caught in.
Notice the speed at which the noise is most audible. Jack up one wheel at a time and try to hit that speed. Do remember though, that if you jack up one wheel and the other is on the ground, due to the inner workings of your differential, the spinning wheel will be spinning with double the speed as your speedometer is showing.