Why do small chunks of felt sometimes come out my 1993 accord ac vents when the heat is on?

This "felt" is a material used to quiet whistling noises as air flows through the ducts. Over time, the material has degraded and become brittle, allowing bits of it to break off. If you spray some toluene into the interior of the car with the system set to recirculate (not fresh air) this may help. Do not inhale any of the toluene however as it isn't good for your health.