My wife took her car to the auto-shop today. The mechanic asked her what seemed to be the trouble with the car and she explained that she thought that one of the cylinders was not working. "Maybe a faulty spark plug," she said. "You'll have to start the engine," he replied, "I have to see" and he pointed to his ear. She started the engine and he immediately -- and I mean, immediately!-- noticed that there was a problem with at least one cylinder.
She thought for a moment that this was a precipitated gesture but later he repeated: "I can hear through the exhaust tube if the cylinders are all working properly.”
Hearing is believing.
Not long ago I read that new technicians at the French Citröen factory are trained to distinguish a “vocabulary” of over 20 sounds that identify multiple engine malfunctions.