Sorry it took so long to get back to you.  I'm in the process of moving.  It could be going bad.  Depends on the quality of the fan, hours of use, and if it was just a sucky built fan to begin with or not.  Also cleaning the dust could inadvertanly fix it by moving the bearing just enought that it's seated on a good area internally.  Then as time goes on and the fan runs it slips back to a bad area and starts clicking.  My above mod is the way I deal with all dust in my computer.  The fan sets behind the filter so even the fan is protected.