World Trade Center - Firefighters Hear Explosions

This footage is from the 9/11 (2002) film. 9/11 was nominated for and won multiple awards including an Emmy for Outstanding Non-Fiction Special.
This documentary was directed by the so called Naudet Brothers, Jules Naudet and Gédéon Naudet and also James Hanlon.

Filmmakers James Hanlon and the Naudet brothers were originally filming Tony Benetatos, a probationary firefighter of the New York City Fire Department assigned to the Engine 7/Ladder 1/Battalion 1 Firehouse on Duane Street in Lower Manhattan with the intention of making a film about the "probie's" first experience as a firefighter.

On the morning of September 11, the firehouse, under the direction of Battalion Chief Joseph Pfeifer, was called out on a reported "odor of gas" at Church and Lispenard Streets. Jules rode with Pfeifer to investigate, while Gedeon stayed behind at the firehouse with the "probie."

As the Battalion 1 firefighters examine the supposed gas leak, American Airlines Flight 11 flew overhead. Turning the camera to follow the plane, Jules taped one of only three known recordings of the first plane hitting the North Tower (Tower 1) of the World Trade Center, the others being a video shot by Pavel Hlava and a sequence of still frames taken by Wolfgang Staehle.

Source: Wikileaks: https://en.wikipedia.org/wiki/9/11_(2002_film)