Once the big leaks were sealed, I moved on to the branch ducts. Basically, these ducts take some of the air from the main metal ducts work and deliver it to individual registers in different parts of the house. They are flexible ducts and attached with clamps.
Many of these too were leaking.
I adjusted 5-6 of them and sealed some with mastic (a white latex-like glue that works awesome). Some were fixed with he metal tape.