Here we have some NYFD fire fighters describing what it was like when the towers fell.
The commonly held opinion is that the towers were brought down like a controlled demolition.
Floor by floor, explosions can be seen as puffs of smoke and these firefighters describe the towers collapsing and the sound of sequenced explosions as they run from the falling building.
This New York firefighter above, describes the noise as they moved away from the falling tower. Boom Boom Boom. And these NYFD firefighters describe explosions at the tower as it fell.