Butterfly valve - 650i MSport exhaust
There is a vacuum-actuated butterfly valve in one of the two exhaust openings on each side.
I think this is what makes that delightful sound in sport mode when the gearbox shifts up. (In another thread someone called it a 'booming sound')
It sounds open on a cold start too. When else does it open?
A few (poor) photos of the valve and the actuator:
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