REP-TAT-P-1214-02 Installing the knock sensor of cylinders 4 to 6
WARNING: This page is about a different car, the 2018 BMW 340i, 2017 BMW 340i, and 2016 BMW 340i. However, it is still accessible from the selected car via links, so may be relevant.
Clean the contact surface of the knock sensor at the crankcase.