Why does the engine make abnormal noises?

[PeterDella]

Engine abnormal noise is caused by engine knocking. The following is extended information about knocking: 1. Detonation causes engine knocking. 2. Detonation is an abnormal combustion phenomenon in the engine. 3. Detonation occurs during the compression stroke. 4. When the engine is in the compression stroke, both the intake and exhaust valves close, and the piston moves upward. 5. The upward movement of the piston compresses the air-fuel mixture. 6. The pressure and temperature of the compressed air-fuel mixture increase. If the air-fuel mixture ignites prematurely, it causes detonation. 7. The combustion of the air-fuel mixture pushes the piston downward while it is moving upward. 8. This affects the engine's power output and fuel economy. 9. Severe detonation can lead to connecting rod deformation or piston cracking. 10. Engine detonation is caused by using inappropriate gasoline or carbon deposits in the combustion chamber.

[LincolnLee]

Last time my neighbor's car made a clicking noise during cold start, and after opening it up, we found the timing chain was loose. Nowadays many cars use chains instead of belts, but when worn out, excessive clearance can cause a metallic clanging sound like hitting sheet metal. When the valve lifters lack oil, they also produce a metal knocking noise, like hitting the engine with a wrench. The scariest is the clattering sound from worn connecting rod bearings—that's the crankshaft and connecting rods grinding dry. Keep driving like that, and you might need an engine overhaul. I often tell my friends, an engine is like a heart: piston ring leaks sound like a hissing whistle, deformed turbo blades scream like a whistle, and a failed oil pump turns the whole machine into a clattering choir. If you hear metal grinding, shut it off immediately—fixing a bearing costs a few hundred, but a major overhaul will make you cry with bills in the tens of thousands.

[McEthan]

Old Zhang's pickup truck in our fleet makes a rattling noise when accelerating, and the inspection revealed that the exhaust pipe hanger was broken, causing resonance. In fact, engine abnormal noises can be divided into two categories: mechanical noise and airflow noise. Loose flywheel screws produce a clattering impact sound during startup; a worn-out generator bearing sounds like sandpaper grinding against metal; a cracked spark plug ceramic body creates a ticking electrical noise. The fuel system is also troublesome—when the fuel injector is clogged, the fuel pump whines mournfully, and a leaking high-pressure fuel line sounds like a gas stove igniting. When repairing cars, we often use a stethoscope to listen to the cylinder walls—excessive piston side clearance sounds like soybeans shaking in a metal can. The most dreaded issue is aging valve stem seals—when engine oil seeps into the combustion chamber, it produces a gurgling sound like boiling porridge.

[DeJordan]

I've been running an auto repair shop for fifteen years, and every morning someone comes in complaining about strange engine noises. One female owner said her car made a clattering sound when accelerating—turned out a screw from the air filter box had gone missing, leaving the plastic cover drumming against the intake pulses. Another older gentleman's vintage car sounded like a tractor during cold starts because all the engine mounts had deteriorated, letting the engine dance freely. Worn universal joints on the driveshaft produce a rhythmic clicking noise, especially noticeable during turns. A failing water pump bearing whines like a cat in heat, while a slipping belt chirps like a little bird. Just the other day, a modified exhaust car had a droning cabin resonance at low RPMs—turns out the aftermarket exhaust lacked proper hangers. If you hear odd noises, don’t panic; recording the sound for your mechanic is the smartest first step.

[VonEmma]

Last year while driving on the highway, the engine suddenly started making a rattling noise. A check at the service area revealed that the oil dipstick wasn't inserted tightly, and the intake pipe was sucking in air. Actually, abnormal noises should be analyzed by scenario: A squealing sound during cold starts is most likely due to belt aging - if it disappears after the engine warms up, there's no need to rush to replace it; Metallic knocking sounds when climbing hills are often caused by engine knocking, indicating it's time to switch to higher-octane gasoline; Gurgling sounds from the engine bay when the AC is on usually mean the compressor bearings are failing; Clunking noises on bumpy roads require checking the exhaust hangers. My car once made a thudding sound after going over speed bumps - turned out to be loose screws on the engine guard plate, fixed for just thirty bucks. Turbocharged cars especially need to watch out for hissing air leak sounds, which indicate cracked intake piping.

[EloiseRose]

When I was working on classic cars, I encountered worn crankshaft thrust washers - there'd be a clunking noise whenever I pressed the clutch. Actually, diagnosing abnormal sounds has its tricks: use a long screwdriver pressed against different engine parts to listen. Ticking sounds near the cylinder head indicate excessive valve clearance, while swishing noises around the oil pan suggest bearing issues. High-frequency metallic scraping points to accessory bearing problems, whereas low-frequency rumbling may indicate carbon buildup in combustion chambers. In the used car market, vehicles with damaged valve lifters are common - they sound like sewing machines ticking at idle. Once when repairing a BMW straight-six engine, clogged hydraulic valve lifters caused piston slap during cold starts, but the noise disappeared after warming up. I recommend immediate inspection for regular abnormal sounds, while occasional noises can be monitored first.