I'm going to guess regenerative braking. It's engaging and charging the system.
(Me, 2015 435i with 80k miles).
If you want to experiment with it NOT being on... try turning off the auto start/stop feature (little button under the Start/Stop Engine button) and then the next time you come to a stop, shift into neutral and hold the car at the stop in neutral versus using the brake and leaving it in drive. Shift back into drive when the light turns green.
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BMW CCA Member | 2015 BMW 435i RWD, Stage 1 Bootmod3
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