What Causes the Rattling Sound When Starting the Car in the Morning?
2 Answers
Reasons for abnormal noise during engine startup: Engine Mount Rubber Deterioration or Loosening: The engine mount rubber is the rubber block placed between the engine and the car frame. Its purpose is to reduce vibration and provide cushioning during engine operation, as well as to secure the engine in place. Once the engine mount rubber deteriorates or becomes loose, the engine may start to shake, and in severe cases, a "clattering" abnormal noise may occur. Excessively Rich Air-Fuel Mixture: When the engine's air-fuel mixture is too rich, symptoms include difficulty accelerating after startup, uneven engine operation; black smoke from the exhaust pipe, accompanied by a "clattering" abnormal noise from the engine, sometimes with backfires; reduced engine power and increased fuel consumption. "Piston Slap" Abnormal Noise: Due to poor lubrication conditions, excessive wear of the piston and cylinder walls leads to the clearance between the piston and cylinder wall exceeding the normal tolerance range. During engine operation, the piston skirt impacts the cylinder wall, producing a crisp "clattering" sound, which is usually more noticeable at idle or low speeds.
Last time at the auto repair shop, the mechanic told me that the most common cause of the rattling noise when starting the car on winter mornings is the oil not circulating yet. After sitting overnight, all the oil settles at the bottom, leaving metal parts like pistons and bearing shells in the engine to grind dry, sounding like sandpaper scraping a pot. It's especially noticeable in cars using the wrong oil grade—recommend checking the dipstick for viscosity. Here's a lesser-known tip: if the noise persists beyond 20 seconds, be alert—it might indicate a VTC actuator failure. My Accord had this part replaced last year.
