It's probably dry/worn bearings for fan motor. Take the dimensions,check voltage and search for a new fan on -bay, rs-components or elsewhere online.

Air quality sensor fan
 

That is the air quality sensor fan.....I m not sure what it does but if. You don't like the sound there are two connectors...one green an one black....try are near the back of the cage towards the top covered in foam....disconnect and the motor stops running and you have peace and quiet.

This was the case on my prefacelift...I can't remember if it is the same on the facelift but have a look.

Thanks Ranj, I think the sensor serves the same purpose but there is nothing obvious to disconnect to disable it. At least I know that if it does get truely bad I can take it apart again and remove the whole motor and fan. Its soldered onto the circuit board, but looks straight forward to permanently disable.