lastly but not least... unless the car is accelerating the biggest noise is the road noise (pavement vs vulcanized rubber). Last I checked, electric cars don't solve this problem. If you're relying on Engine noise to determine if a car is coming, you're already fairly screwed.
This is really just not true. I spend a lot of time on a road bicycle during the warmer months, so cars are passing all the time; and hybrids in electric mode are dead silent coming up behind you, frequently scaring the crap out me and my team mates. All other vehicles make audible noise. Not sure if it's the engine, the engine fan, or what. But you definitely hear it on internal combustion engines and not on hybrids in full electric mode.