When does the fan noise happen? Many cars run the radiator fan(s) after the car is shut off until the coolant temperature is reduced down below a certain point, so that could explain a fan noise. If you're hearing a fan after it's been off for some hours then it's probably something else, but a fan that continues or comes on not long after turning off the car is most likely normal. It shouldn't continue for too long; more than a half hour is almost certainly a problem.
Have you opened the hood when you hear the sound and looked around to see if you can find the source?