I finally discovered the cause of the sound that I was trying to describe in post 2. And as the OP described, it does originate from the left side of the car, between the charging port and the windshield (in other words, from in front of the driver, probably comes from the other side in countries that have right-side-drive cars).
The Niro has an electric motor that drives a hydraulic pump that pressurizes the brake lines. If you want to hear what it sounds like: park in a quiet place, probably a good idea to put the driver's window down, turn the car off, and then pump the brakes a few times. I didn't hear anything at first. Along about the fourth or fifth time I pressed the brake pedal, I put my ear near the open window, and then I could hear the sound that I'd previously noticed on occasion from outside the car. I wish I'd paid enough attention to think about whether I was hearing it when I pressed the brake or when I released it, but I didn't, so if you try this, maybe you'll focus on that question and update this thread with your observation.
, I have the impression that this pump runs frequently while driving (actually while braking), but since I've not noticed it while driving, even in EV mode (it's a very subtle sound in the cabin), if this is the same sound that you're hearing and it's highly noticeable to you while you're driving, then either my hearing is going bad or your pump is overly loud. If you conclude that your pump is overly loud, the first thing to do is check your brake fluid level (low brake fluid might cause the pump to be loud, but more importantly, might be a sign of an imminent safety hazard). The second thing to do might be to check out another Niro (perhaps on a dealer's lot) and pump the brakes and see if it seems as loud as your car or not. If you think your pump is louder than the average Niro, that could be an indication that the pump in your car is on the verge of failing, which could result in a very unsafe driving situation. I hope that's not your situation.
Reflecting back on my prior post, it now makes sense to me that I sometimes hear this sound when I open the driver's door. I'm assuming that opening the door "wakes up" the computer, the computer notices that the brake fluid pressure is low, and it turns on the pump on the assumption that I might be about to get in the car and put my foot on the brake in preparation to drive. But this begs the question about what causes the brake fluid pressure to sometimes get low when the car is off? I suspect that the answer is that there are several electrically controlled valves in the brake system that separate the high pressure side from the low pressure side: if any one of them isn't seated exactly perfectly and it allows even a few ml of fluid to weep past it over a period of several hours, this might be enough to lower the pressure to the point that the computer will want to start the pump the next time it wakes up. So long as it's intermittent and you're not actually seeing the brake fluid level drop in the reservoir, I don't think it's anything to be concerned about, but if I began hearing this sound every time I got in the car, I might question if maybe one of the valves wasn't working correctly and if so, maybe that indicated that my brakes weren't going to work as well as they should.