1999 Volkswagen Jetta foam flying out of vents

This a VW Jetta, 82000, Auto, 4 cyl. Every time the fan is turned on (air or heat) bits of black foam come flying out of the vents. Sometimes as big as 2 inches square. I think it is more pronounced when the recirculating button is pushed. Can you tell me what is breaking appart in the system and how can I replace it. Thanks