Is it only doing it while driving and not engine rev dependant?
If so, I would look at the drive shaft carrier bearings.
Can you get the car solidly on stands or a lift and get the wheels spinning? Once everything is moving, and the noise starts, you can walk around it and try and pinpoint the cause.
Please be careful with this method and have someone in the car you trust that can stop the wheels if something bad happens.
If you cannot replicate the noise in the air, it may be wheel bearing noise under the wiegth of the car.