What Causes Knocking Sounds Under the Chassis After the Car Shuts Off?
2 Answers
Reasons for abnormal noises under the chassis after the car shuts off: 1. Cooling sounds from the catalytic converter: The normal operating temperature of the catalytic converter is 400-800°C. After shutdown, its operation gradually stops, and the temperature decreases, causing thermal expansion and contraction of internal metal components, resulting in abnormal noises. 2. Cooling sounds from the exhaust pipe: During cold idling, the exhaust pipe can reach temperatures above 200°C. Under high load or when unburned fuel ignites in the exhaust pipe, temperatures can rise even higher, leading to thermal expansion and contraction noises, often originating from the front end or flexible joint of the exhaust pipe. 3. Turbocharger cooling sounds: Normally, exhaust gases can reach 600-700°C, accelerating turbo rotation and friction-generated heat. After shutdown, thermal expansion and contraction become pronounced, causing abnormal noises.
I've been repairing cars for over a decade and often encounter the issue of chassis noises after engine shutdown. These sounds are mostly caused by thermal expansion and contraction of the exhaust system - when the engine turns off, the exhaust pipes cool rapidly, causing metal contraction that produces pinging sounds. However, it could also indicate loose suspension components, especially worn control arm bushings in older vehicles that release stress when shutting down. The braking system should also be checked - loose caliper bracket bolts or poorly retracting brake pads can create abnormal noises after parking. The catalytic converter may also produce sounds as its internal ceramic substrate contracts during cooling. If a brand-new car makes such noises right after being driven home, it's likely normal; but if sudden metallic scraping sounds appear or the noise persists unusually long, immediate inspection of chassis bolts and suspension linkages is required.
