What Causes the Intermittent Buzzing Sound During Car Idling?
2 Answers
Engine shaking with a sound resembling a tractor is referred to as "engine backfire." Engine backfire occurs when, after the piston completes its work in the cylinder, the exhaust gases that should be expelled through the exhaust pipe cannot exit due to a certain malfunction—either the exhaust valve has closed prematurely, or the gases cannot be expelled quickly enough. Consequently, the exhaust gases escape from individual cylinders in the cylinder head, the crankcase, the carburetor, the throttle valve, and other areas, commonly known as "backflow" or engine backfire. Below are common causes of engine backfire: 1. Overly lean air-fuel mixture: Possible causes include faults in the fuel system or intake system. Fuel system faults mainly result from insufficient fuel injection by the injector, which can be due to low fuel pressure or a clogged injector. Intake system faults primarily stem from excessive air intake, caused by a malfunctioning air intake sensor or a leaking intake manifold. 2. Issues with the ignition system: Mainly insufficient ignition energy (due to excessive resistance in high-voltage wires, damaged ignition coils, or insufficient power supply voltage) or faulty spark plugs. 3. Excessive ignition advance angle: Main causes include improper CKP (crankshaft position sensor) gap, loose CKP, damaged temperature sensor, engine load, or a faulty ECU. 4. Significant deviation of the ignition advance angle from the correct position: Symptoms: If the engine exhibits both backfire and popping sounds, and the issue is severe, it is often due to incorrectly connected cylinder high-voltage wires. If the issue is less severe but occurs intermittently and seems to follow a pattern, it is usually caused by cracks in the distributor cap, leading to cross-firing between cylinders.
I've been driving for over 20 years and have encountered several instances where the car made intermittent humming noises at idle. The first time was during summer when I noticed the sound after turning on the AC. It sounded like a fan starting and stopping repeatedly. Actually, it was the cooling fan dissipating heat – it automatically turns on when engine temperature rises and stops when the temperature is adequate, creating that intermittent humming. Later during inspection, I found the fan blades were slightly deformed, and replacing them solved the issue. Another time with an older car, there was an air leak in the intake system causing turbulent airflow, making the engine produce humming sounds when inhaling air. I had to check the air filter and connections for proper sealing. Cold starts in winter also often cause this, especially when the engine oil is too thin with insufficient lubrication, resulting in louder vibrations. If the humming is accompanied by vibrations, it's likely due to worn engine mounts that need prompt replacement – otherwise highway driving feels unsafe. In short, never underestimate these sounds; early troubleshooting can prevent major repairs.
