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 mucho fasto...