What exactly is the problem ?

I recently noticed a very strong sewage smell coming from my fridgefreezer, there's no blockage in the drainage hole & no food which has turned, a couple of days later the smell was gone, only to be replaced by another strange smell similar 2 an overheating smell, this smell is still there but not as bad as it was, the appliance is kept in a well ventilated cupboard which is continuously opened throughout the day, any advice would be greatly appreciated, thank you so much for your email re this problem.