Glad to hear others have noticed this as well.
Our first trip down to LA and back this happened. It was at the Harris Ranch Supercharger. At approximately 100 miles charged the noise started to get louder and louder. I eventually stopped the charge because I thought something was seriously wrong. We tried another bay at the Supercharger and it started up again. This was 3 days into ownership so I didn't want to mess up my new car.
The car had about 130 miles of charge and the next Supercharger (Gilroy) was about 90 miles away. No problem, I thought, and we headed on our way. After about 10-20 minutes of driving, I realized that the 90 miles was as the crow flies not the real 111 miles of roads to drive. At this point I had range anxiety (first and only time). It was late at night, the wind was blowing hard against us, it was cold and to top it off, I knew the last leg of the drive had hills... Energy's worst friend. If the hills were in the middle of the trip I wouldn't have worried because I knew I could get back some of the energy with regen. Long story short"er", I drove the car extremely slow. 60-65mph on a freeway with a 70mph limit. I pulled into the Gilroy SC with over 30 miles of range meaning I beat the EPA range. Only problem, the noise happened again. I got enough charge to get home and then left.
The next day I brought my car into the local SC. They charged the car all day to make sure everything was behaving normally. When I picked up the car, they said everything was fine and that it is normal to hear that noise when the pack is warm and you are charging at a high rate.
Still a little skeptical, I was always on the watch for that sound again. On my second trip to LA, it happened again on the way back at the same location. This time I let it go and the sound slowed down over time. I noticed that the car was blasting out hot air from the front driver side wheel well. I have a similar video of the noise but never had a chance to post it.
I think for those that haven't heard the sound, you either haven't driven the car far enough at one time (multiple superchargers) or you get out of the vehicle when it is charging and the sound is gone by the time you are back. Anyway, glad to hear that my experience was normal (except for the extremely slow drive home on that first trip).