So here's a thread about exhaust 'rattle':
http://f30.bimmerpost.com/forums/sho...d.php?t=972489
As I looked through posts about the flappers there is sure a lot of confusion about what the flappers are for.
They are for increased horsepower at higher engine speeds.
I'm not sure what RPM they are programed to 'open' but the Corvette has a similar system that is called a 'Dual Mode Exhaust'. On the Vette the 'flappers' open at 3500 RPM and provide an extra 5 HP. As the power and RPM increase obviously the airflow through the system increases as does back pressure. Opening the valves at high power, reduces back pressure and increases HP because in the open mode the exhaust has a direct path through the muffler without going through the baffles and also increases the sound level.