I heard it again tonight, under different conditions, and it's kind of disconcerting.
I went out for lunch earlier today, came back around 2 PM and parked in the garage. My EV range was down to about 10 miles. Around 6 PM, I went back to the garage and plugged in the L1 charger. There were a few of the usual clicking sounds as I plugged it in, and then a humming sound started and didn't stop. It reminded me of a fuel pump sound. It was probably about 80F in the garage and it occurred to me that it might be a battery cooling fan, so I opened the rear hatch, lifted the access panel so that I was looking at the traction battery housing, but the sound didn't seem to be coming from there. When I got down on the floor and listened from under the rear bumper, it was pretty clearly coming from somewhere under the car. I'm pretty much convinced that it was a fuel pump sound.
After a few minutes of this, I unplugged the charger from the wall, and then from the car. A moment later, the humming stopped. I reconnected the charger and the humming didn't start again.
I think it's probably been about three weeks since I last drove in a way that required the ICE to start, and I wonder if that might be a factor. It has occurred to me that it might not be good to run the ICE that infrequently. Still, if it really was a fuel pump that I heard, I'm having a hard time imagining how plugging in the charger could have triggered it to come on.
Interested to hear any ideas that people might have.
2018 Niro PHEV Gravity Blue