My Hoover fridge freezer is stinking from somewhere on the OUTSIDE. Help!?

My Hoover frost-free fridge freezer, which we keep in a cupboard under the stairs, is absolutely stinking. Can't identify the smell, but its a kind of revolting savoury smell that's getting worse and becoming more rotten and unbearable each day. It's making the whole house stink. There is no trace of any smell inside either section, and we can't find the source of the smell on the back of the appliance, but it's definitely coming from it somewhere. Has anyone got any ideas at all? Please?!