If you hear the fan being activated on a cool day, or if the fan is in operation after even a short run, there’s a good chance that the level of coolant in your car has dropped, or there’s a fault elsewhere, such as with the thermostat.
It’s worth letting the car’s cooling system cool down so you can accurately check the level of fluid in the expansion tank – it should be between and H and L mark on the side of the tank.
If the level of coolant is low, it’s worthwhile checking the state of the car’s rubber cooling pipes. These lead from the radiator to the expansion tank and from the car’s engine to the radiator.
Vibrations caused when the car is moving can make the pipes wear if they happen to be rubbing against other components, and if there are any bulges in the pipes you should replace them straight away and bleed the system.