This:
The car will spend an hour or two each day charging the 12V battery, assuming you haven't been driving it or charging the HV battery recently, and during this time it will make the humming noise. If you use certain third-party apps, it may do this more often (and that's not a good thing). (If you have, the solution is to change your password - logging out of the apps won't help.)
During this period, a little over 14V will be applied to the 12V battery (it's injecting a controlled current into the battery during this time). When it's done, the voltage will drop (DC-DC off, so 12V battery now is supplying current), to around 13.6V, and subsequently be allowed to decay to just under 13V (about 12.8V for my car), and then the cycle will repeat.
If you're driving the car, or it's waiting for a download or something, or you're in sentry mode, and the battery doesn't need charging, the DC-DC will "maintain" at 13.4V.