Here are some of the most likely culprits:
* Rod knock: This is a very serious problem. It means a connecting rod bearing is failing, causing the rod to hit the crankshaft. This will likely require a rebuild or engine replacement. Rod knock is often accompanied by a progressively worsening knocking sound as the engine warms up. If the knock is consistent and doesn't change much with engine temperature, it's less likely to be a rod knock.
* Main bearing knock: Similar to rod knock, but involves the bearings that support the crankshaft itself. This is also a very serious problem requiring major engine repair.
* Wrist pin knock: The wrist pin connects the piston to the connecting rod. If this is worn or damaged, you'll hear a knocking sound. This is also a serious issue that requires engine work.
* Low oil pressure: Lack of sufficient oil pressure can cause parts to rub against each other, creating a knocking sound. Check your oil level immediately. Low oil could be due to a leak, a failing oil pump, or simply neglecting oil changes.
* Valvetrain issues: While less likely to cause a very loud knock immediately on startup, worn or damaged valves or lifters can produce a ticking or knocking noise that's more pronounced when cold.
* Loose components: Although less likely to cause a *loud* knock immediately upon startup, it's possible a loose component is making noise. This is less severe than the other possibilities.
What you should do:
1. Do NOT drive the car. Further damage is almost certain if you continue driving with this knocking sound.
2. Check the oil level immediately. Is it low? Is it dirty? Use the dipstick to check.
3. Listen carefully to the noise: Does it change as the engine warms up? Does it change with engine speed? This can help pinpoint the source.
4. Take it to a mechanic ASAP. A qualified mechanic can diagnose the problem properly. Ignoring it will likely result in far greater expense later.
The loud knocking sound strongly suggests a serious internal engine problem. Getting it checked by a mechanic is essential to prevent more extensive and costly damage. The earlier it's diagnosed, the better the chance of a less expensive repair (or at least knowing the extent of the damage).
