It's scary when a microwave oven keeps the fan running for a while after the time is up and even while you have the door open. The noise is exactly the same as when the oven is doing its job.
You feel like the microwaves are still being generated and will blast your face when you open the door to take out your bowl of delicious Zuppa Ortolana.
This is bad UX. The sound produced should be clearly different when the microwaves aren't present. Run the fan at lower RPM (but for a bit longer), for instance.