I've seen some photo's posted here. I'll snap a few, reduce the file sizes, and post them.
I see what looks like a thin layer of metal, but I swear it's not the INTERNAL surface. It's like behind it.
Originally posted by t527ed
in slab ductwork 40 yrs old is probably metal or transite. never saw pvc or fiberglass in anything that old around here. at least take a flashlight and mirror, pop out a few supply registers and have a look. you will know if it needs to be cleaned.