- Piston slap: This is a common issue in Subaru engines, especially those with high mileage. It occurs when the piston makes contact with the cylinder wall, causing a knocking sound. Piston slap is typically more noticeable on cold starts and goes away as the engine warms up.
- Connecting rod bearing: A worn connecting rod bearing can also cause a knocking sound. This is a more serious issue than piston slap and can eventually lead to engine failure.
- Valvetrain: A problem with the valvetrain, such as a worn camshaft or lifter, can also cause a knocking sound.
- Knock Sensor: A faulty knock sensor can also trigger the knocking sound. This sensor detects engine knocking and adjusts the ignition timing to prevent damage to the engine.
If you are experiencing a knocking sound on startup in your 2004 Subaru Outback, it's important to have the vehicle inspected by a qualified mechanic to determine the exact cause of the issue and ensure the long life of your engine.
