Engine knock at startup

My car 2004 Chevrolet Classic with 193,000 miles on it started knocking. I parked it. The radiator reservoir was empty so I filled it. Cranked the car and the sound began to quiet bit did nit go away. Now when the car is started it knocks hard for a second at startup then dulls to barely noticeable. It knocks more noticeable in reverse. Oil was changed after the knock began as well. What could cause this?

My car has 193000 miles.
My car has an automatic transmission.

Experienced mechanics share their insights in answering this question :
Hello there. In most cases, an engine knocking sound at start up is caused by hardware inside the cylinder head of the motor that is loose or is not properly lubricated. As the engine starts, and oil is circulated to the top of the motor, the knocking will subside. This type of noise can also be caused by a loose belt or pulley. As such, it is probably best to have a professional mechanic come to your location and complete a car is making a noise inspection, so they can determine what is causing this noise.

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When you first start it in the morning, it takes a bit of time for the oil pressure to build allowing the oil to circulate properly throughout the motor. This can cause a bit of knocking and is somewhat normal as long as it does not last too long and is not excessive.
Engine Knock…

The symptoms are especially a problem when the ambient temperature is below 50 degrees F, following a cold start. During these temperatures the piston rocks/ slaps against the cylinder wall creating a knocking sound that many confuse with bearing clearance issues.

The ticking sound is commonly caused by opening and closing valves or rocking arms. This could mean the car is low on oil or there is a problem with the oil delivery system that needs to be repaired. The noise can go away once the parts are properly lubricated.
Pre-ignition is an uncontrolled combustion event which occurs before the spark event. Knock (pinging) is the actual noise that can be audibly heard if detonation is bad enough. Most OEM knock sensors are basically microphones tuned to listen for a specific frequency to detect this noise.
Pre-ignition (or preignition) in a spark-ignition engine is a technically different phenomenon from engine knocking, and describes the event wherein the air/fuel mixture in the cylinder ignites before the spark plug fires.
That dreaded clicking noise can usually be traced to the battery, and the fix could be as simple as a jump-start or tightening a cable. Just one click, though, probably means the fault lies with the starter motor (more on that later).
An engine with low oil or low oil pressure can make a ticking noise due to a lack of lubrication. If you are low on oil, it may indicate either oil consumption or an oil leak, both of which should be addressed as soon as possible. If you have low oil pressure, you will likely get a warning on the dash.
Knocking occurs when fuel burns unevenly in your engine. When everything is going as it should, and the cylinders have the correct mix of air and fuel, the mixture burns in a controlled, progressive manner. After each cylinder`s air/fuel mixture burns, it should create a small “shock wave” in your engine.
Engine knocking often occurs when the air-fuel mixture is incorrect. To counter this, you can add an octane booster in the mix. This can help ensure proper octane rating, which can then stop the knocking. The recommended minimum octane level in the US is 87.
Engine knock results from the miss-timing of the combustion process between the injector firing and piston movement, resulting in a knocking sound from the engine. This symptom indicates that more components than just the injector are now involved.
Engine knock is usually triggered by low octane fuel, high compression ratio, high intake temperature, or advanced ignition timing. Pre-ignition, on the other hand, happens when the fuel-air mixture ignites before the spark plug fires, creating a single flame front that compresses the air-fuel mixture ahead of it.
The symptoms of a piston failure can include engine noise (rattling or knocking noises while the engine is idling), oil burning, misfiring and loss of power.
Knocking can damage the surface of the piston, the cylinder walls or the crankshaft bearings, all of which are expensive to repair. Modern computer-controlled injection systems can correct your fuel mixture to prevent knocking, but at the cost of engine performance.
Once an engine starts to knock, the rod can fracture without warning. It could be the next time you start it in your driveway, or it could keep going for six months. Eventually though, the engine will blow and you`ll be stranded somewhere.
Knocking can damage the surface of the piston, the cylinder walls or the crankshaft bearings, all of which are expensive to repair. Modern computer-controlled injection systems can correct your fuel mixture to prevent knocking, but at the cost of engine performance.

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