ich_iel but American

In the US at least, it seems like they have settled on a sort of sci-fi hovercraft whirring sound that modulates in pitch a bit so that it captures your attention, but is pretty inaudible from more than a few carlengths away. Honestly pretty similar the noise level of an idling modern ICE.

It seems to fit the bill of being recognizable without being obtrusive.

I would guess that makes (especially high end) are going to quickly find their own signature sound, which sounds a bit obnoxious, but inevitable.