What Causes Abnormal Noises When Starting the Car Engine?
2 Answers
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.
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.
