Lacking internal combustion engines, electric cars are naturally silent, yet they need to produce a noise for both driver feedback and pedestrian awareness. BMW's designers are especially concerned with the former for their M-branded electric vehicles; the high-performance M-series cars--yes, even the i4--are meant to deliver thrills, not boring hums.
Thus they hired composer Hans Zimmer, known for scoring epic movies like Blade Runner 2049, Dunkirk, Inception, The Dark Knight trilogy, etc., to design the M-series' electric vehicle fake engine sounds. Here it is, and if you just want to hear the sound and not the explanation, fast-forward to 1:36 in the video:
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BMW has given Zimmer's creation a flowery name, and discusses it almost like an oenophile describing a wine:
"In the basic setting, BMW IconicSounds Electric employs a strikingly transparent timbre with spherical components to convey the basic character of an electrically driven BMW brand model.
"An immersive and pleasant atmosphere is created initially in the BMW i4 in COMFORT mode. But when the driver steps on the accelerator, the direct relationship between how the car is being driven and the sounds it makes is immediately revealed: the sound experience deepens as load and speed increase. In SPORT mode, in particular, the car's aural spectrum is more dominant and powerful, the sound providing a constant commentary on the driving situation.
"…Only when the vehicle is travelling in ECO PRO mode is the acoustic feedback reduced to nothing. Here, the silence of the drive system underscores the driver's decision to adopt an ultra-efficient driving style."