* Low Engine Oil Pressure (Most Likely): This is the *top suspect*. When the engine is cold, the oil is thicker. If the oil pump is weak, or you have low oil, the pressure might not be sufficient to adequately lubricate the engine components, leading to knocking. Once the oil warms up and thins, the pressure increases, and the knocking stops. Check your oil level immediately. If it's low, add oil and see if the knocking improves. If it's at the correct level, consider an oil pressure test.
* Worn Piston Rings/Cylinder Walls: This is more serious. As piston rings wear, they allow combustion pressure to leak past the pistons, creating a knocking sound, especially when the engine is cold and components are tighter. This knocking often persists even when warm, but it can be more pronounced when cold. A compression test is needed to diagnose this.
* Worn Connecting Rod Bearings: Similar to piston rings, worn connecting rod bearings can cause knocking, particularly noticeable when cold. This is a serious issue that needs immediate attention to avoid engine failure.
* Lifter Tick (Less Likely but Possible): Hydraulic lifters can sometimes make a ticking or tapping sound when the engine is cold. While usually a ticking, rather than a knocking, it can sound similar. They often quiet down once the engine oil warms up and properly lubricates them. This is less likely to be a significant *knocking* sound.
* Valvetrain Issues: Worn or damaged components in the valvetrain could also contribute to knocking, although this is less common to disappear once warm.
What to Do:
1. Check your oil level and condition immediately. Low oil or oil that's very dark and dirty is a strong indicator of a problem.
2. Listen carefully to the location of the knock. Is it coming from the top end (valvetrain area), the bottom end (crankshaft/connecting rods), or somewhere else? This helps pinpoint the problem area.
3. Perform a compression test. This is crucial to assess the condition of your cylinders and piston rings. A significant drop in compression in one or more cylinders points towards piston ring or cylinder wall wear.
4. Perform an oil pressure test. This will tell you if your oil pressure is sufficient at both cold and warm operating temperatures.
5. Consult a mechanic. If you're not comfortable performing these tests yourself, take your Tahoe to a qualified mechanic for diagnosis and repair. Ignoring engine knocking can lead to catastrophic engine failure.
Important Note: Driving with a knocking engine can cause significant damage. Minimize driving until the issue is diagnosed and resolved. The longer you wait, the more expensive the repair will be.
