Fan running during cooktop usage
This is normal. The microwave oven’s cooling fan (on some models), which is separate from the vent fan, automatically comes on during microwave oven operation to cool the microwave oven. It may also automatically come on and cycle on and off to cool the microwave oven’s controls while the cooktop below is being used. It may stay on for up to 1 hour to cool the microwave oven. When this occurs, the vent fan cannot be turned off.
Fan running during cook function
This is normal. The vent fan will automatically turn on during any cook function (on some models), acting as a cooling fan to protect the microwave.