I also installed a L2 station (Clipper Creek HCS-40, 32A) last week for my 22 day old leaf and I can hear the same high humming sound.
I found a youtube video of a 2013 doing the exact same thing. It does sound like a fan (cooling maybe?).
https://www.youtube.com/watch?v=OKGXzWLNmqs"...
This was my experience at Gwinnet Nissan.
Yesterday I called Gwinnett Nissan and spoke with Griffin Lee. He emailed me the following lease quote for the 2015 Leaf Model S and setup an appointment/test drive for the following day at 9:30am EST. I also requested pricing on the model SV, but...