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Hi my name is Robert...
This sounds like the insulation that is used between the roof and the inside ceiling. Sometimes it can come out of the vents when the AC is going. What I would do is cover the areas below the vents and then get an air nozzle and blow out all the ducting really good. That should get most of it out, blow through each vent and then repeat going the other way. Sometimes when the vents get routed out it will go through the installation and if it does not get cleaned out it can sit there.... Hope this helps, IF you have more questions or concerns please feel free to conact me.... Robert