Oren writes:

A careless bunch of workers. No more duct cleaning for hire. Seeing
the method, they used I can make the tool and do it IF I was ever
inclined to do so.

What does the tool look like? I've never seen it done, but I have
access to both compressed air and a shop vac. I know they drill small
holes in the duct for access, but not what they put through the holes.

Dave