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The AWD drive cannot be disabled. There is a different transfer case than the 4WD models.
In A4WD, the generic electronic module (GEM) varies the torque split between front and rear drivelines by controlling the transfer case clutch. Under most conditions, the GEM activates the transfer case clutch at a minimum duty cycle (percentage of time the clutch is turned on) which allows for a slight speed difference between the front and rear driveshafts which normally occurs when negotiating a corner on dry pavement. When the rear wheels are overpowered, the GEM detects this slip condition, and the duty cycle to the transfer case clutch is increased until the speed difference between driveshafts is reduced. In this manner, the GEM can redirect engine torque to the front wheels when the rear wheels lose traction yet still allow operation in the A4WD mode on dry pavement.
It is usually normal that a small bang will be felt when the front axle kicks in. It may do this more than it should or be even louder if your tires tires are not worn evenly or are different size. As much as 1/2 inch in tire circumference can cause a lot banging from the transfer case.
How do your tires look?