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The order of the cylinders for all BMW 6-cylinder engines is 1 through 6, starting at the front of the engine. In other words, cylinder #1 is at the front of the engine (behind the radiator) and #6 is at the rear of the engine (close to the windshield).
Bank 1 and Bank 2 indicates the different exhaust streams that the sensors are located in. Bank 1 refers to cylinders 1, 2, and 3; Bank 2 is cylinders 4, 5, and 6. Bank 1 is the exhaust manifold toward the front of the car and Bank 2 is the exhaust header immediately aft of that. I think that hissing sound you hear is the secondary air pump kicking on during a cold start.