Worked in opposite side of Pentagon from impact. Only felt a rumble in the chest as in a movie-theater type explosion, no noise from the impact, or concussion that far away in that large and solid a structure.
Ran bravely away some hours later and I had a first hand look at how a modern city will collapse - Mad Max-like traffic free for all, overwhelmed infrastructure, etc.
Incredibly quiet in the DC suburbs; not a sound of air traffic when that is normally a non-stop background noise.
Poignant in later days to see the normally ceremonial Old Guard outfitted in biohazard suits and going through the piles of rubbish looking for DNA material.