Why does the car make a rattling sound at idle when hot but not when cold?
2 Answers
Here are the reasons for the "abnormal noise" during cold starts: 1. Oil pump: After the car has been parked for a day, the engine's lubricating oil has mostly flowed back into the oil pan. During a cold start, the oil pump cannot immediately establish oil pressure, and oil films cannot form between components, especially the valves in the cylinder head (upper part). Whether it's hydraulic lifters or mechanical rocker arms, there will be some valve clearance, resulting in a rattling sound. 2. Charcoal canister solenoid valve: Besides the oil issue, another component that can cause a rattling sound is the charcoal canister solenoid valve (purge valve). Its function is to open the solenoid valve between the charcoal canister and the intake manifold after the engine starts, allowing the gasoline vapor adsorbed by the charcoal canister to be drawn into the cylinders by the vacuum in the intake manifold to participate in combustion.
The other day my car made a ticking noise at idle when warmed up, but was completely silent when cold, which really puzzled me. When the engine heats up, components expand, possibly increasing valve clearance or causing insufficient oil supply to hydraulic lifters, leading to metallic knocking sounds. It's like how people tend to stretch more in hot weather—thermal expansion increases friction. When cold, the oil is more viscous and lubricates better, so the noise is minimal. I recommend first checking if the oil level is adequate and whether the oil is clean; dirty oil has poor flow properties, and when hot, dropping oil pressure can cause noise. Sticking piston rings can also cause this, common in older cars. Don't drive recklessly when you hear such noises—safety first. Take it to a repair shop to have them use a stethoscope on the valve area; don't try to save money by guessing and risk bigger problems. Regular oil changes and maintenance can prevent this issue.
