zanussi fridge freezer smells?

Fridge has started to smell when you open the door, it smells putrid, defrosted the fridge freezer and cleaned the appliance and all shelves etc., put everything back in and shut the door. On opening it again the smell was there again. We cleaned it out several times more, still the same. it is frost free the pipes down to the tray on top of the condensor are clear. could this be a gas leakage ?