Switch off the Extractor Fan
Over-the-range microwave ovens often incorporate an extractor fan that removes steam, smoke and odors from the cooking surface below. The fan makes a gentle humming noise while it is running. You may also hear a "whooshing" sound as air is sucked through the ducting. A button is usually on the microwave that lets you turn off the extractor fan.
Wait a Few Minutes
Some microwave ovens feature a cooling fan that automatically switches on when the underside of the microwave reaches a particular temperature. The fan helps cool the underside of the microwave and the internal components to prevent damage. The fan turns off when the stove is turned off, and the temperature returns to normal.
Listen to the LCD Display
Listen if the humming sound is coming from the LCD display, where the time appears. The transformer that powers the clock makes a low humming noise at all times. You should not hear the hum over normal background noise.
Unplug the Microwave
Disconnect the power to the microwave, and the hum should stop immediately. The sound might not be coming from the microwave if you can still hear it with the oven unplugged. Keep the microwave unplugged, and contact Whirlpool if the noise returns with the power supply.