Some kind of fan noise on the inside of Vauxhall Vectra 2ltr GLS?

Hi i have just bought a V reg Vectra when the engine is running you can hear a whine like its a fan on the inside of the car, when you excelerate it sometimes goes away but will return and allso when you take the key out it keeps running but slowes down to a stop after about 10 secs can anyone point me in the right direction as how to stop/find whats doing it