A common form of magnesium silicate which was used in various construction products due to it's stability and resistance to fire. Asbestos exposure, by inhaling loose asbestos fibers, is associated with various forms of lung disease.

See also: EPA Asbestos in Your Home, Q&A: Encapsulating Asbestos, Q&A: Does My Insulation Contain Asbestos, Q&A: Is This Asbestos?, Q&A: Asbestos Floor Tiles, Q&A: Asbestos Ceiling Tiles

Some heating ducts with damaged asbestos tape.
The hole maybe a rodent access point.
It is also a defect in the duct system.