Black junk from air vents

Black graphite feeling material has started to blow out of the air vents. It sticks to the seats and ceiling. Most is very small & powdery but there have been a few pieces up to 1/4 inch wide and 1 inch long. I know it's not coming from the cabin air filter. My repair shop thinks it is some type of coating used inside the heater/air box (whatever you call it) that is starting to degrade and come off. They don't have a solution. Any ideas?

2003 330i