I don't have the BSW upgrade, but the sound change has to do with the amount of road noise, which the car compensates for (I believe it is speed and noise variable)

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2006 BMW X5 4.8is Lemans Blue/Beige/Dark Maple
-Nav, Comfort Seats, Heated Steering Wheel,
Rear Airbags, AUX Input
2009 BMW M3 Space Grey/Black/Carbon
-all options