strange noise from engine

Hi, not sure how to describe this question as im not mechanically minded so I will do it in my own words and hope you understand, there is a strange noise coming from my renault megane 1.6 car its a w reg so about 15 yrs old, the noise is coming from the bolted unit just above the cam belt on the left hand side of the engine, ive tried to find out what this unit is but no luck, dont know if its the cooling fan or or what but it clunks and whirs very strangly, there is no difference to the car perfomance while im driving or when the car is idling. any ideas as to what it could be???Please.