I have a seven year old Homark Built Under fridge which regularly (every 6-8 weeks) gives off a bad odour which smell like something rotting. I clean the fridge frequently, including keeping the drain hole clear, but despite this the smell returns. Unfortunately as it is built in I haven’t been able to reach any other parts and indeed can find nothing in the owners manual that requires me to do so to clean the fridge. What is the problem and how can I get rid of the smell on a more permanent basis?