The inner fan may have developed faulty bearings and if this happens the noise will be very loud, like a plane landing in your loungeroom. You need to replace the fan.

Another reason is the fan blade itself may have 'moved' on its axis or is slightly out of position, it maybe touching the fan bracket, so slightly moving the fan or bracket to different position can be an easy fix. If you are looking at this while unit is runnig remember to be careful as the connections to the fan do contain 240V. It's best to have power off and spin fan yourself to see if it is hitting any part of the bracket.

Part
If you need a new fan, enter details below, the part number is SK135-FAN-INNER, price is $119.00