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 do contain 240V.

Part
If you need a new fan, enter details below, the part number is BR125-FAN-INNER, price is $119.00