4 Wheel Drive/SUVs: 2005 lexus rx 330

Question
Hi-
The car is well maintained but the problem comes from the front end. There is a distinct humming noise coming from the front end and it varies according to the road surface we're traveling on. All our traveling is on highways. I've had new front end wheel bearings installed and also new tires put on and yet the noise persists. I don't know what to try next. Could it be a transmission problem? The car shifts fine and there is no evidence of leaking. I'm frustrated because mechanics seem to follow the process of elimination approach. Any ideas?
Thanks

Answer
Terance,

This sounds like road noise from the tires. If you are running an all terrain type of tread, as opposed to street tread, this is not uncommon.  Issues other than the tires, could be a differential noise, or transmission noise.  You may have the fluids changed in the transmission and differentials, to see it this helps.
I still think that it is a tire issue, as the sound changes with the road surface.

Carl