BMW ALPINA B3s exhaust flap does work.

B3Sのエキゾーストフラップ、動画撮影に成功しました。 詳細は、「みんカラ」で!

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R32 Exhaust Flapper explained
I had my stock Exhaust off the 2008 Volkswagen .:R32.Showing the "Flapper valve" on the muffler.Some people do whats called the "Flapper mod"this has nothing to do with that.I am just explaining how the flapper works what it does.

BMW E36 328i exhaust silencer flap operation

e90, e92, e93 BMW Exhaust Flap Rattle Fix
This happens to a large amount of these cars. If you drive a e90, e92, e93 or maybe other ones, this will definitely help you at one point. This mod also makes your Exhaust sound very nice (a tiny bit louder)

Capristo RS5 exhaust flaps