That box has three distinct components.
One is an AC power distribution panel, filled with circuit breakers... but they won't hum.
Another is the DC power distribution panel, filled with fuses... and they won't hum either.
In the bottom is the converter/charger, which converts 120V AC power (from the shore power cable) to 12V DC power for the battery and everything that runs on 12V DC... so it's almost certainly the converter/charger.
The converter/charger does have a fan, which should run only when the converter/charger gets warm enough to need cooling; it shouldn't run all of the time. Electronic parts can also hum, but that usually isn't audible with a modern converter/charger working properly.