As you can see above, many different types of wraps and tapes were used on HVAC ducting.  These materials often contain a very high percentage of asbestos, and should always be tested and appropriately handled.

Here, white asbestos tape is shown on the outside of HVAC ducting
Asbestos tape is shown here within the HVAC duct registers
This duct has been completely wrapped with sheets of asbestos containing materials
This HVAC supply register has been coated with sheets of asbestos containing materials, which have become damaged leading to potential fiber release