The new car accelerates normally, and there is a slight shooting sound when it is idling in neutral. What's the matter?

The problem you mentioned should be caused by the internal structure of the muffler. No problem. It's normal imagination, so don't worry. When you are accelerating rapidly, the throttle opening increases, the air intake increases rapidly, and a large amount of air enters combustion, which leads to the rapid elimination of burned gas and unburned gas, which only increases the exhaust pressure of the exhaust pipe. When exhausting quickly, the muffler pressure will be too high, resulting in air resistance vibration. When the red light is idling, the throttle opening is small, the air intake is small, and the gas produced by combustion is also small. When removing gas, the exhaust gas is reduced by the internal mechanism of muffler, but when the exhaust gas accumulates to a certain amount, it will be discharged in large quantities, causing exhaust gas vibration. This is what you call knocking. During normal driving, the intake and exhaust volume will not increase sharply, but will be eliminated smoothly under the action of exhaust pressure, so there will be no exhaust vibration.