What causes the clunking sound when the car starts moving?
3 Answers
The clunking sound when the car starts moving is caused by friction between the brake pads and brake discs, which is normal and nothing for the owner to worry about. The vehicle's braking system consists of the master cylinder, vacuum booster, wheel cylinder, brake pads, and brake discs. When the brake pedal is pressed, the brake pads clamp onto the brake discs, converting kinetic energy into heat energy to slow down the car. The vacuum booster provides assistance when the brake pedal is pressed, making it easier for the driver to apply the brakes. Upon opening the engine hood, a black metal disc can be seen below the windshield on the driver's side - this is the vacuum booster. In front of the vacuum booster is the master cylinder, which has a reservoir for holding brake fluid.
Last time I drove an old Jetta, I encountered a similar situation, and later found out it was due to a loose universal joint on the driveshaft. The most common cause of a clunking sound when starting is aging chassis components, such as cracked bushings on the lower control arm causing metal-to-metal contact, or increased clearance due to worn CV joints. Loose stabilizer links can also produce a metallic knocking sound at the moment of starting. For four-wheel-drive vehicles, it's also important to check for loose transfer case mounting bolts. Actually, the simplest thing is to first check if the wheel bolts are tightened properly—I have a friend who forgot to tighten the bolts after changing tires, resulting in a loud clunk when starting. It's recommended to visit a repair shop as soon as possible to have the car lifted and inspected, especially to pinpoint whether the sound is coming from the front compartment or the middle of the chassis. Don't ignore such abnormal noises or wait until the parts are completely damaged before fixing them.
With 12 years of experience running a repair shop and handling hundreds of cases of 'clunking' noises, I've categorized them into three main types: The most critical is loose chassis connecting components—when control arm bushings crack, metal parts collide directly. Next is wear in power transmission parts—excessive clearance in CV joint ball cages causes jerking during initial acceleration. Suspension component misalignment also triggers abnormal sounds—when shock absorber mount bearings seize, spring release produces impact noises. The most dangerous case I've encountered was a loose tie rod end ball joint causing wheel wobble during acceleration. I recommend three diagnostic tests: Listen for sounds during light throttle application on cold starts, identify noise locations when starting with wheels at full lock, and observe vibration changes during coasting followed by sudden acceleration. These details provide crucial clues for technicians.
