I believe you are not hearing the cooling fan, but the air charge pump. It happens when you first start the car and very shortly when stopping sometimes. It is very quick and a bit loud. It doesn't just happen when stopping, that's just when you can hear it. If you open the hood, you will see a black, cylinder on the driver's side in the front of the engine compartment. Listen while the hood is open, you'll find it. Research it on this site, there is plenty of discussion about what exactly it does.