if it was a wire hitting fan why does it only do it on face vents ? and if a wire was hitting the fan at speed 3 or 4 it would not tick, it would be a continuous noise.
IMHO the centrifugal fan is too well sealed for anything to hit the blades, try closing of the centre vents one at a time as it may be one of the guide vents loose and flapping when the air goes past it