2008 Cadillac SRX humming noise

Several possibilities, they include wheel bearings(any of the four), transfer case noise(if this is truly all wheel drive), front or rear(most common) differential noise. First to check would be if you can duplicate this noise when lifted and run up to speed. Next would be to check fluid levels in the differentials. If this does not lead anywhere, it may require the use of a listening device like Steelman's Chassis Ear. This has sensors that can be placed on the vehicle in chosen locations and roadtesting can be done while listening to each of these sensors one at a time through a switch box. The vehicle should still be under warranty, were you aware of that?