First, check to make sure that the reflectors on the top of the tailgate are not bowed. If they are they contact the hatch causing noise.
Second, Check all of the plastic interior pieces on the tailgate including the two interior lights to make sure they are not loose.
Third, instead of adjusting the stoppers, rough up the contact surface of the stopper and the mating landing on the hatch using some sandpaper. Noise is created by the two surfaces sliding against each other, by roughing the surfaces, it creates more friction. Good Luck

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2013 X5 3.5i M Sport
2004 M3 cabriolet SMG