could anyone tell me why the old G.E. refrigerator my mom has (1968 vintage) makes this noise sometimes like someone belching? It really made for some interesting jokes while at the dinner table with the family as a kid. This fridge is one of the old ones with the fresh food compartment up top, and the freezer on the bottom with a pull out drawer. Does the defrost system of this fridge reverse the refrigerant cycle like a heat pump?