Diplomat ADP 5330

Hi, my diplomat cooker makes a loud buzzing noise (like a transformer buzzing)when mains power is applied to the cooker, but with no oven / grill etc physically switched on. It appears to be coming from the top left hand corner, behind the green power on light. Does anyone have any ides as to what it may be and how to solve it. The buzzing noise does get quieter after about half an hour, but is still very prominent, hence the wife switching the mains off after every use. Thanks