Why does the engine sound fluctuate between loud and quiet?

[KennedyLee]

When the engine is idling after warming up, it's normal for the sound to fluctuate between loud and quiet, as long as the variation isn't too pronounced and the engine doesn't shake. Below is relevant information about car idling: 1. The purpose of car idling: Idling is an operating condition of a car, referring to the engine running in neutral gear. The engine speed during idling is called the idle speed. The idle speed can be adjusted by modifying the throttle opening size, among other methods. Idling means the engine is 'producing power without doing work.' When the engine is running, if the accelerator pedal is completely released, the engine is in an idling state. 2. Causes of unstable idling: Direct causes refer to mechanical parts being dirty, worn, or incorrectly installed, leading to changes in cylinder power output, resulting in an imbalance of power among cylinders and causing unstable idling. Indirect causes refer to abnormalities in the engine's electronic control system, leading to poor combustion of the air-fuel mixture, making it difficult to balance the power output among cylinders and causing unstable idling.