Truck Knocking

This is a Video from a 2013 Ford F-150 with the 5.0 engine that is making a knocking/flapping noise. You have to listen carefully because of the other outside noise. It made this noise for over 2 years and 40,000 miles. had it checked multiple times at the dealership and they were unable to locate the noise. The other week the serpentine belt shredded on the truck and had to be replaced. Noise disappeared so the only thing I can assume is that the serpentine belt had a work or bad spot that was causing the noise.