What kind of noise is it, is it squeaking, a grating noise, high or low pitch, and is it continuous or intermittent?
There are a number of reasons the most likely issue is the fan\motor drive shaft.
If it comes on when on in standby this is probably a faulty button on the control panel, the loud noise is most likely a bearing in the motor which is starting to fail.
This isnt something you would be able to fix yourself, and would suggest you take it to an approved service centre, you can find your nearest one by clicker here.
The noise will get louder as the fault develops and due to the nature of the fault I would be reluctant to leave the unit on unsupervised, as if left could become a fire hazard.
Please let me know if you need anything clarified.