What Causes Abnormal Noises When Starting the Car Engine?

[StJackson]

The reasons for abnormal noises when starting the car engine are as follows: 1. Loose fasteners. The vibration generated during engine operation causes certain components to loosen, resulting in knocking sounds. For example, loose flywheel mounting bolts or loose connecting rod bolts can cause abnormal noises. 2. Deformation or damage of individual components. Abnormal noises can be caused by deformation or damage of certain parts. For instance, a bent connecting rod can lead to piston slap, while broken valve springs or a fractured crankshaft can also cause abnormal noises. 3. Improper assembly, adjustment, or repair. Incorrect assembly, adjustment, or repair can lead to improper clearance between components. Examples include excessively tight piston pin assembly or improperly adjusted valve clearance, both of which can cause abnormal noises.

[DanielFitz]

Abnormal noises during car startup can range from minor to serious issues. A common cause is the accessory belt slipping or aging, especially during cold starts in winter, producing a 'squeaking' sound like singing. My neighbor's old Tiguan had this exact problem—it became quiet immediately after replacing the belt and tensioner. Another possibility is the slow retraction of the starter motor's flywheel, creating a harsh metal grinding noise. For turbocharged cars, a leaking blow-off valve can also produce a 'hissing' sound. The most concerning issue is internal engine bearing problems, such as a damaged water pump or alternator bearing, which can cause a rhythmic 'humming' noise. Identifying the sound's location is crucial. It's advisable not to ignore it—quickly record the noise with your phone and let a professional mechanic diagnose it.

[McJayce]

Don't panic about startup noises - 9 times out of 10 it's just oil not circulating properly. My 80,000km Corolla had this exact issue last month - it would make a 'ticking' sound for 3-5 seconds on cold starts. The mechanic explained this is typically caused by insufficient oil pressure in hydraulic lifters, especially common in older vehicles. Checking the dipstick revealed oil level below minimum - switching to full synthetic oil immediately improved it. However, if you hear metallic knocking sounds, it could indicate serious issues like loose piston pins or worn crankshaft bearings requiring engine disassembly. Pro tip: Listen with the hood open during cold starts - if the noise disappears after warming up, it's likely belt or bearing related.

[RuthMarie]

Engine noises can be categorized into high-frequency and low-frequency. A short 'click' sound may indicate improper engagement of the starter gear. Last time at the 4S shop, a BMW 3 Series made a loud 'clunk' noise upon starting—it turned out to be a worn flywheel ring gear. Metal scraping noises should raise suspicion about the timing chain; my mechanic friend mentioned that stretched chain links can rub against the guides. If there's a 'clang' sound like a hammer strike, it's likely engine knocking, often caused by using low-quality 92-octane fuel. The most alarming is a 'crunching' sound, which could mean a broken balance shaft. If you hear any rhythmic abnormal noise, it's best to shut off the engine immediately and call for a tow truck.