Noise When running

We have an 8cyl AWD Premier Model. The mechanics have looked and looked and cannot isolate it. We took it to the dealer, they have no idea either.
The car makes a humming/whining sound when its idling and when you are driving it, never gets any louder but is always there. It sounds like a power steering pump going out when you turn the wheels, Thats the noise it makes, The mechanics have promised me it is NOT the pump because it does it ( makes the noise) weather you turn the wheel or not.