You do not specify what year. With the air filter on the E36 (approx 1992-1999) you must remove the glovebox. After the glovebox is removed you will see the air filter on the left side (on an RHD car)
The microfilter is the filter for the air that enters the cabin of the vehicle.
http://brokenappliance.blogspot.com/2008/06/replacing-bmw-e60-microfilter-cabin-air.html
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A cabin micro filter on a BMW1 series is an air filter for the inside of the car. It is under the dash and catches dust and other airborne particles.
On the E39, there's 1 on each side at the rear of the engine compartment. They're located inside plastic boxes.
The BMW was founded by Franz Joseph Popp. The BMW 3 series was first launched in Leipzig, Germany. The Manufacturer of the BMW 3 series was founded by the German Auto Maker BMW. The first BMW 3 series was manufactured in 1975.
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The 2005 BMW 3-Series has 24 valves.
The 2001 BMW 3-Series has 24 valves.
The 2008 BMW 3-Series has 24 valves.
The 2007 BMW 3-Series has 24 valves.
The 2009 BMW 3-Series has 24 valves.