That creaking is the sound of torque being applied to the rear suspension and diff bushings. When you shift to drive or reverse, with the brakes on, the motors torque cannot spin the wheels, so it just loads up the motor mounts/diff bushings/suspension bushings. This is normal unless the sound...
To reduce this tendency, you can fit a larger front antiroll bar. Alternatively, you can use a smaller rear antiroll bar.
As an added benefit, if you go with the larger front bar, you will also find that your steering will be more heavy in general. The Bmw dual axis geometry of the front...