Propane... what you hear is the propane pushing air out of the line. Put a lit match next to the burner and wait for the flame.
Is the battery
connected? Is it charged. Sometimes there is a switch on the front of the power panel to switch from ac in to battery/DC power
But that said you don't need DC for most refrigerators if they is over about 10 years old. Running the refrigerator
on DC to cool will kill your battery