Some report hearing a high pitch engine sound like that of a missile or fighter jet.
http://killtown.911review.org/oddities/911.html • 9/11 - Witnesses at the Pentagon report that the aircraft that crashed into the Pentagon sounded and/or behaved like a missile.
► Richard Benedetto, USA TODAY reporter
"...saw the plane slam into the Pentagon. "It sounded like an artillery shell. It hit on the west side of the building, near the helipad." - USA Today, 'I fear for my daughter' (09/11/01)
► Michael DiPaula, 41, project coordinator Pentagon Renovation Team
"Suddenly, an airplane roared into view, nearly shearing the roof off the trailer before slamming into the E ring. 'It sounded like a missile,' DiPaula recalls." - Baltimore Sun
► Lon Rains, Editor at Space News
"With the Pentagon to the left of my van at about 10 o’clock on the dial of a clock, I glanced at my watch to see if I was going to be late for my appointment.
At that moment I heard a very loud, quick whooshing sound that began behind me and stopped suddenly in front of me and to my left. In fractions of a second I heard the impact and an explosion. The next thing I saw was the fireball.
I was convinced it was a missile. It came in so fast it sounded nothing like an airplane. Friends and colleagues have asked me if I felt a shock wave and I honestly do not know." - Space News, Eyewitness: The Pentagon (06/30/05)
► Tom Seibert, 33, Pentagon network engineer
""We heard what sounded like a missile, then we heard a loud boom," said Tom Seibert, 33, a network engineer at the Pentagon. "We were sitting there and watching this thing from New York, and I said, you know, the next best target would be us. And five minutes later, boom."" - Guardian, 'Everyone was screaming, crying, running. It's like a war zone' (09/12/01)