My parents have just brought a new oven, the forst in 20 years and it jas a top oven/grill. We know the cooling gan comes on once the grill and oven have been turned off. When we used the top oven to cook in it sounded as though the cooling fan came on. This is not a fam assisted oven. Was wonderingnof this is common or if there is a problem with the oven?