Loud? Why has no one mentioned the 1-11 yet?!
Did not hear 1-11 in person. Did hear the first generation jets, 707, 727, etc which delivered ear-piercing shrieking sounds.
The one that took the prize for me was Concorde taking off. It was a truly visceral experience. It wasn't just sound, the shock waves thumped against one's chest and caused a throbbing sensation. I experienced this in the car park at LHR, so I wasn't particularly close, as such effects drop with the square of distance. I can only imagine what it was like to be closer.
I was close (enough) to a Concorde taking off. At CDG, I was sitting in an airplane, the airplane stopped taxiing, and the Captain came on the public speaker system, apologizing for a delay, explaining that the runway is occupied for Concorde takeoff, and suggesting we take a look (we were on a taxiway perpendicular to the runway, Concorde was taking off right to left, in front of us. I was on the port side, so could only see very little to the right). It was not the noise that surprised most, however (I was used to Tu-154 and An-24 at the time; airplane noise was a given). Our little 737 was... Not shaking, not rocking... I would say dribbling under us. (sorry, English is not my first language, not even a second. I might be somewhat limited in the vocabulary to describe this short-amplitude, not strong, but oscillating... jumps). When the noise arrived en force, the Concorde was already to the left, gradually rotating.