Why is black dust is coming out of my heater/AC vents

[johng422]

Black dust and little bits of what looks like foam insulation is coming out of the vents of my otherwise pristine 1991 350SD. It accumulates on the carpets and sills next to the front seats.

Makes a real mess!

Has anyone else had this problem? Is there something I can do about it or do I just have to wait until all the insulation is gone.

I was thinking of ingesting some kind of goo into the ductwork that would seal this stuff in place, but I'm afraid that anything I would use would be very toxic and/or gum up the working parts and sensors.

Suggestions, sympathy???

John