I had to tell a customer that his muffler bearing was bad and making the rattling noise he was hearing… I took him out to the car cause I was not about to say that in front of other people…
BMW active muffler I think it is called, Napa shows the muffler list price at about $2400.00, special order…
Think of the muffler as having two exhaust tips coming out. at the inside end of one of the tips is a butterfly valve that opens and closes (like a drive by wire throttle body) to increase backpressure at idle and under light throttle. I believe one of the benefits is that it improves low end torque. If you’re right behind one at a traffic light, you can see it in action.
On my customers BMW the bearing had blown out and the flap was just flopping around inside making the rattling noise…
I have it on my scat pack challenger. they call it active exhaust. when I start my vehicle on a cold start it is fully open. then it must shut a bit and quiets down a little as it warms. in sport mode it stays open more. and in track mode it will stay fully open. even partially shut it has a nice rumble.