I was re reading previous breads and an anon mentioned a jet noise that sounded like a roar and lasted for more than a few minutes. Think high altitude and high speed, but picture a tunnel through the sky instead of a regular plane flying through it. Where as a regular aircraft generates resistance in a delta pattern from the nose outward, these have a spiral generated ahead of them. Delta = boom when speed of sound is exceeded. Spiral = speed / resistance exist simultaneously. Hence the roar you hear is the same as a boom starting at one end of a tunnel and being heard at the opposite end. Except in this case, the "tunnel" in the sky is expanding as the aircraft moves and the continuous roar is the echo down the spiral to the point where it dissipates. (Opposite end)