Can anybody help with this, any previous threads on loud roaring noise from engine bay only when air-con is on, help please...

Hi All, my name is Conleth, I have just bought a 2003 RR 4.4 L322 in green, lovely car. When the air con is on I get a loud roaring noise from the area around the main fan belt, any ideas, soon as I turn off the air con the roar goes away. Many Thanks, Conleth.