I've noticed on my E46 2003 325xi that when I turn the car off and step
out I can hear what sounds like some kind of hum that somethings is
still working, then there's a escape of some kind of air pressure
followed by a squeak of soemthing closing at the rear of the car.  It
can take anywhere from 15 seconds to a few minutes after I've shut down
the car.  Never bothered me, but always curious.

While cleaning my car today (albeit pointless with this pollen in North
Carolina it seems) and cleaning the exhaust tips I noticed that one of
the tailpipe has a damper connected to a vacuum actuator above it.  It
seems this damper (or flapper, whatever you want to call it) stays
closed most of the time while running (and it's the source of the sound
I mentioned above as it opens).

Anyone know what this does and why?  Most curious...

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Joe Morris
E46 2003 325xi