Why an engine might experience knocking noises when it's cold ?

 There can be a few reasons why an engine might experience knocking noises when it's cold. Here are some possible explanations:

1. Low-quality fuel: If you're using low-octane fuel, it can cause knocking noises in the engine. The fuel burns too quickly and can cause premature detonation in the cylinders, leading to knocking sounds.

2. Carbon buildup: Over time, carbon deposits can accumulate on the pistons and cylinder walls, causing hot spots that can ignite the fuel prematurely. This can result in knocking sounds, particularly when the engine is cold.

3. Worn bearings: Bearings are responsible for reducing friction between moving parts in the engine. Over time, they can become worn or damaged, causing increased friction and noise. This can be particularly noticeable when the engine is cold.

4. Timing issues: If the timing of the engine is off, it can cause knocking noises. This can happen if the timing belt or chain is worn or if the timing is not properly calibrated.

5. Incorrect oil viscosity: Using the wrong type of oil or an oil with the incorrect viscosity can cause problems with lubrication and result in knocking sounds.

If you're experiencing knocking noises in your engine, particularly when it's cold, it's best to have a mechanic inspect it to determine the root cause of the issue. They can provide a proper diagnosis and suggest any necessary repairs.