This is due to resonance, meaning a build up of waves or oscillations all in the ‘same place’.
A jet engine is loud – the sound travels out from it in all directions at the speed of sound. The sound is vibrations in the air. But if the jet is travelling at the speed of sound then it keeps up with the sound waves that it makes and just keeps adding more and more to the vibration of the air. So the amplitude of the wave is magnified enormously.
Usually jets accelerate through the sound barrier – they try not to travel for a long time at the speed of sound in case the air vibration damages the aircraft.
When the sound barrier was first broken (by Chuck Yeager – action man!) the engineers were a bit concerned that doing so would destroy the airplane.
This happens because the air ahead of the vehicle cannot move quickly enough, so pressure builds up. Eventually this leads to a shock wave, creating a “boom” which trails behind the aircraft (or car!).
Growing up in Suffolk I occasionally experienced these booms, as well as the eerie sight of a large jet at low altitude slient overhead. The deafening bang which followed moments later was almost enough to cause an accident in my trousers!
Sorry to take so long to answer your question. I am trying to get as many votes as possible on Friday, so I took some of the time to make this video blog. Please have a look, and see if I can persuade you to vote for me!