I would love a quiet car, and all kinds of things can be done to accomplish this. zero gaps between the doors and pillars of the car, a smooth underside, no whip antenna. A lot of these things can be retrofitted to existing cars, and are all likely going to be engineered into the model 3, since these are also factors in drag generated by the car. besides that, acoustically shielding the motor noise from the passenger compartment, undercoatings, and low noise tires go a long way.
This is the same concept as my trimmer. most trimmers are so loud, it's annoying to use, much less for the neighbors. with my trimmer, it was already pretty quiet to begin with, but the string is actually quite loud, too. I found a string that generates very little noise, and you have a trimmer that just sounds like a small 4 stroke running very quietly.
My point is, all the little noises add up, and detract from the overall experience. I have a saturn ion, and in a mission to reduce drag and noise, experimented with a lot of things, and wind noise is very easy to almost eliminate with just a little engineering. I have been waiting for another car to implement another cadillac/insight type rear tire skirt. this is yet another way you can reduce noise. The type of fan used for the HVAC makes a huge difference, there are so many types, and they all have different noise signatures.
As someone else said, sound isolation is expensive, and heavy, but anything you can do to reduce the generation of noise is gold. For example, from the le5 2.4 L ecotec engine to the lkw 2.5 L ecotec. GM completely re engineered how the engine was laid out to reduce noise and vibration.
Anyway, with the low drag of the car, and EV powertrain, I think the 3 might just be a contender for the quietest car out there.