What Causes the Engine Sound to Fluctuate Between Loud and Quiet During Hot Idle?

[Ella]

When the engine is idling while hot, the sound may fluctuate between loud and quiet, but as long as the variation is not very noticeable and there is no shaking, it is considered normal. Below is an introduction to car idling: Introduction to Car Idling: Idling is a working condition of the car, referring to the engine running in neutral. The engine speed during idling is called the idle speed. The idle speed can be adjusted by modifying the throttle size, among other factors. Idling means the engine is "producing power without doing work." When the engine is running, if the accelerator pedal is fully released, the engine enters the idle state. Causes of Unstable Idling: Direct causes refer to mechanical parts being dirty, worn, or incorrectly installed, leading to changes in cylinder power and resulting in an imbalance of power among the cylinders, 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 among the cylinders and causing unstable idling.

[Craig]

My car often has fluctuating idle sounds after warming up, which could be caused by various factors. I think the most common issue is a problem with the ignition system, such as aging spark plugs or carbon buildup blocking them, leading to engine misfires. Alternatively, an unstable fuel pump supply or uneven gasoline injection can cause inconsistent combustion in the cylinders, naturally resulting in unstable sounds. I've encountered this a few times, mostly due to neglected regular maintenance. Additionally, a small valve or vacuum leak in the idle control system can disrupt RPM control, making the engine "breathe" unevenly. In short, this kind of problem shouldn't be delayed, as it can accelerate wear and even cause emission issues. I recommend first checking simple components, like replacing spark plugs or cleaning the throttle body. If that doesn't work, take it to a professional repair shop immediately to be safe. Using quality gasoline and regular maintenance can greatly reduce such troubles.

[IsaacDella]

When my car is just warmed up, the idle sound keeps fluctuating between loud and quiet, which really worries me. I think the main reason might be excessive carbon buildup in the engine, clogging the combustion chamber and causing uneven power output; or it could be dirty fuel lines or air filter affecting the air-fuel mixture ratio. I once drove a friend's old car that had similar symptoms, and it turned out to be the oxygen sensor sending faulty data, causing the ECU to constantly adjust fuel injection. This abnormal sound is not only annoying but also reduces fuel efficiency, and prolonged neglect might even affect driving safety. What I fear most is sudden breakdowns on the road, so it's best not to ignore it—immediately check the spark plugs and clean the throttle body. If there's no improvement, have a technician perform a full diagnosis right away to prevent minor issues from turning into major repairs. Regularly replacing the air filter and using high-quality gasoline can prevent problems before they occur.

[VanCarmen]

My car's idle noise fluctuates when it's hot, and the reason is likely in the details of the control system. For example, unstable fuel pump pressure or a stuck idle motor can cause the engine speed to fluctuate repeatedly. Additionally, if sensors such as the temperature or throttle position sensor malfunction, the ECU may receive incorrect signals and misjudge the fuel injection amount. I've researched that carbon buildup can also interfere with combustion stability. All these factors cause the sound to vary in intensity. When addressing this issue, priority should be given to checking whether the ignition coils or spark plugs are worn out, and cleaning the intake passages can prevent further problems.

[Javier]

My car has irregular noises during idle warm-up, which is mostly caused by minor issues. I suspect it could be due to aging ignition coils or carbon buildup on the spark plugs, resulting in intermittent sparking; or a clogged air filter causing an imbalance in the air-fuel mixture. I tried cleaning the throttle body and idle control valve myself, and it was fixed with a small cost. If the problem persists, then check the fuel pump or vacuum line leaks—these DIY fixes aren’t difficult. Don’t rush to spend money at the shop; start with simple maintenance, like replacing filters and cleaning fuel injectors every 5,000 km, which can save a lot of trouble. Long-term neglect may lead to higher fuel consumption and engine damage.