Blowing the dirt out

This dirt or fluff is coming out of the blower area of the furnace. If you have a filter in the proper area then the debris you are describing may be from the insulation around the fan where it is located in the furnace. If these bits of debris has the apperance of black fluff then that is whats going on the insulation aroung the fan inside has come loose and is being picked up by the fan blowing it into your house. The way to repair this is to access the fan area of the furnace and remove the loose insulation. Remember to disconnect power to the furnace to ensure system does not turn on while you are removing the debris. Also remember that there will still be some debris coming out of the system for a while once you have corrected the problem due to the debris being in your duct work from the previous problem however it will slowly disapate.

Good Luck on your repairs

Dave