first off, if it is a bi or tri power fridge ( AC/DC/LP ) there are NO moving parts, they are what is called "Absorption Refrigeration"
you won't hear anything come on, and it takes several hours to actually have any real cooling effect, especially if the unit hasn't operated in awhile.
LP is the most efficient operation method, causes the fridge to cool better, AC works almost as well, takes a little longer to come down to temp, DC will suck the life out of a battery in short order, suggested only to be used when connected to power source from tow vehicle, and not let them sit connected and not running for long.
there are many mods you can do to increase efficiency, add a fan to the outside coil and direct air flow across, greatly improves how it works, some folks also add a fan inside the fridge as well to circulate air inside for more even cooling.