The loudest aircraft I've ever heard was not a jet at all but a prop driven military cargo plane taking off with JATO rockets.
My dad was in the USAF in WWII. One day when I was a kid he and I were hanging around the house, and he suddenly got up and ran out into the front yard, saying as he went "There's nothing else in the world that sounds like that". It was a B-36 flying over at a pretty low altitude. He was right; I've never heard anything else that sounded like that.