Possible Causes of the Knocking:
* Rod Knock: This is a very serious problem. It means a connecting rod bearing has failed, and the rod is striking the crankshaft. This will destroy the engine if continued. It's the most likely culprit given the severity of the noise.
* Main Bearing Knock: Similar to rod knock, but involves the bearings that support the crankshaft itself. Also very serious.
* Piston Slap: While less catastrophic than rod or main bearing knock, piston slap is still a serious issue. It indicates excessive clearance between the piston and cylinder wall, causing a knocking sound, especially when the engine is cold.
* Valve Train Issues: A bent pushrod, lifter failure, or rocker arm problem can create a knocking sound, though often it's more of a ticking or clicking than a deep, heavy knock.
* Loose or Damaged Timing Components: Although less likely given the low mileage, a loose timing chain or gear can cause noises.
What You Should Do IMMEDIATELY:
1. Do NOT drive the car. Continuing to drive it with a severe knocking noise will almost certainly cause catastrophic engine failure.
2. Have it towed to a mechanic: A qualified mechanic needs to diagnose the problem. Mention the low mileage and the car's history – this might help them prioritize certain possibilities.
3. Prepare for potential expenses: Depending on the cause, repairs could range from relatively minor (if it's a valve train issue) to extremely expensive (if it's a rod or main bearing knock requiring a rebuild or engine replacement). A rebuild on an older engine might not be cost-effective compared to a used replacement engine, especially considering the age of the vehicle.
4. Get multiple quotes: Obtain quotes from at least two different reputable mechanics before committing to repairs.
Before the Mechanic Arrives:
* Try to capture the sound: If you can safely and briefly run the engine (only for a few seconds!), record the noise with your phone. This can help the mechanic diagnose the issue.
Given the car's low mileage and careful maintenance history, the cause of the knocking is unusual. It's important to find out the root cause to prevent further damage. Ignoring the problem will only lead to a far more expensive repair bill.
