What part of the world are you driving in? I purchased my PHEV last October, so haven't yet had a chance to experience how it behaves in the extreme heat of a Southern California summer. I do occasionally hear a sound coming from somewhere under the hood, maybe on the left side, that could be described as a humming/whirring sound, but I only hear it when the car is stopped. For example, I drove the car a couple of miles yesterday on a fully charged battery, did not plug it in last night, and then in the middle of the day today, I opened the drivers door to throw something in to the passenger's seat so it would be there later when I wanted it, and I hear a humming/whirring sound when I opened the driver's door. That only happens maybe 10% of the time that I open the door, and I'm not sure what it is about. I've never heard that sound when I was actually driving the car though, and apparently I"m pretty astute about car sounds, because I tend to notice that I pick up on sounds that other people in the car totally ignore.
Not sure if this applies to all packages, but in my EX Premium there's an app on the head unit that can run a self diagnostic. The car has to be electrically on when you run it. I still haven't figured out if it is actually potentially useful or not, but you might try to run it and see if it comes up with anything.
2018 Niro PHEV Gravity Blue