A vapor trail appears speeding out from behind WTC1 to the right, and away from the viewer, just after WTC2 is struck:

From a different perspective: "Flight 175" approaches WTC2

101	105	115

As "Flight 175" reaches WTC2, a vapor trail appears on the right side of the screen moving toward the WTC:

121	122	123

A few frames are omitted; then the object seems to appear just above where "Flight 175" was, right next to WTC2, only an instant earlier...

130

The mystery vapor trail emerges to the left of the towers; in the last 2 frames an additional smudge, or a second mystery vapor trail are faintly visible. Note the distinct rectangular appearing bulges half way down WTC2 in the final frames.

140	142	145	150