Are you hearing a humming noise?
A low hum sound may occur when cooking. The cause of this is the amount of energy which is transmitted from the appliance to the cookware. This sound will quiet or go away when the power is turned down.
The noise may also be caused by vibrations in the joint faces between the different layers of certain cookware. It may change depending on the power level, amount and type of food being cooked, try a different power setting or cookware.