Here's why:
* Idle Knock: At idle, the engine's components are under relatively low stress. A worn connecting rod bearing might only exhibit knocking when the oil pressure is low (as it can be at idle) due to insufficient lubrication. The knock becomes less noticeable or disappears under acceleration because increased oil pressure forces the bearing surfaces together momentarily.
* Disappears under Acceleration: The increased RPMs and oil pressure during acceleration provide a temporary lubrication "cushion", masking the knocking sound.
Other less likely possibilities (but still worth considering):
* Loose exhaust components: While less likely to disappear completely under acceleration, a loose heat shield, exhaust manifold, or other exhaust component could create a knocking sound that changes with engine load.
* Engine mounts: Worn engine mounts can cause the engine to shift and knock against the chassis, but this is usually more of a rattle or thud and less likely to disappear under acceleration.
* Valve train: While less probable given the sound description, a worn lifter or rocker arm could be contributing. However, these tend to exhibit a different kind of noise, often a tappet/tick that's more consistent across engine speeds.
It is crucial to NOT continue driving the car with this noise. Ignoring a connecting rod bearing issue can lead to catastrophic engine damage. The bearings are critical components preventing metal-on-metal contact within the engine. Failure can result in a seized engine and require a major, expensive repair.
Immediate Action:
* Have the car inspected by a qualified mechanic IMMEDIATELY. They can diagnose the problem accurately using tools and experience. Further driving could cause severe engine damage.
* Do not attempt to diagnose or fix this yourself unless you have significant experience with internal combustion engines.
The sound you're hearing is a serious warning sign. Addressing it promptly is vital to prevent costly and potentially irreversible damage to your engine.
