In Stealth Mode (only battery power) there is absolutely no noise issue. And from what I understand (from reviews and test drive feedback) there are 3 ways of using Sport Mode, and only one of those has the 'noise issue'.
1. If you manually select Sport Mode for extra acceleration, the engine/generator will produce noise, but this is not considered a problem because it correlates with acceleration. Actually, the whistle and hiss the turbo-engine/generator produces at that moment make the car sound like a 'space ship', 'jet fighter' and 'batmobile' - quoting some positive comments from people who have driven in this mode.
2. If you manually select Sport Mode to charge the battery, when the battery is still charged for 50% or more, the engine/generator will work quietly, on the background, not producing high revs and intrusive noise.
3. If the car automatically selects Sport Mode to charge the battery, which happens when the battery falls below 20% charge, then some (many?) people find the engine/generator too noisy and intrusive. The engine/generator will produce high revs to quickly get the battery to a 50% charge. From that moment on, but this is a guess, it should become more quiet as it has to work less hard to keep it at 50%.