I have a 2015 one, bought it brand new. I had the same issue before, and that humming was annoying the f out of me. It happened the first time when my car got to 2000 miles, then I took it back to Infiniti for service. They told me the same thing, no problem. After many many times of bringing my car back to service (FYI, I went to a different dealership for service after the first dealership wasn't helpful at all. I tried multiple times, and they didn't do anything for me.), and by the time they fixed the issue, my car was already 50000 miles. "LOL, what a joke!!". and now my car is 113xxx miles. As I remember, the problem came from a torque or throttle module. That part has an issue when you drive to certain RPM at certain gear (although is a CVT). Mine wasn't able to cruise at 1200 rpm around 20 mph. It will hum the f out. Also, beware of the radiator fan failure, most likely it will last to about 70k-90k miles I say. Under the hood of QX60 is so packed, there is not much room for cooling. If you had an overheating issue in the future, check the fan, it will first start from the one supports the AC, then the other. You will hear noise when from the beginning, the fan got loose from the motor, it hits the bracket.
Good Luck!!!