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Ok so here's an update:
I took the car to the BMW services and they agree there is definite booming sound when automatically upshifting in SPORTS mode, however they aren't quite sure why it's causing it, but they don't think it's something wrong cause it's doesn't sound like something's breaking or out of place, instead one could possibly enjoy the sound. well, maybe.. they will report the case to BMW headquaters.
I enquired my odd sound to a very experienced mechanic who has been dealing with BMW and other german cars for a long time, and well reputated in a internet BMW community in Korea, and at once he says it's got to be something to do with that dual flap on the exhaust.
It is known that the exhaust flap opens only at certain rpm's and when you accerelerate and rpm goes up, the flap opens. At this time, there is no booming sound. However when you constantly accerelerate, the car shifts into higher gear, and the rpm suddenly drops. The flap that was opened closese, and it creates a stoppage of air or whatever aerodynamics involved, and that's when you hear the booming sound.
It's not suppose to be any harm to the car.
However I wonder if this happens to all the 650i Converts and Coups with dual speed variable flaps or just to some? and would this sound be intended?
I did try to get it on video, but with my iPhone 4, the bass sound was not recorded well and could just barely hear it. In real life, it is quite noticeble.
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