A knocking sound coming from the engine is a car owner's worst nightmare; however, you can actually fix or diagnose many causes of engine knocking yourself. When you hear a knocking noise coming from your engine, here are some common causes and what you need to do in order to fix them.

You're Using Fuel With a Lower Octane Rating Than Your Car Is Designed For

A common cause of engine knocking is using fuel with an octane rating lower than your car is designed for, such as using regular unleaded fuel in a car designed for premium. Fuel with a lower octane rating combusts more easily than fuel with a higher octane rating. The spark plugs in your engine are designed to use fuel with a certain octane rating or higher – using low-octane fuel causes the spark plugs to ignite the fuel too early in the stroke cycle, which results in knocking and pinging noises as the shockwaves reverberate throughout your engine.

Check your owner's manual and see what octane level your car is rated for. If you accidentally put fuel with a lower octane rating than necessary in your car, you'll need to stop driving it to avoid damaging your spark plugs. The problem is easily fixed by purchasing octane booster from any auto parts store – you simply add it to your fuel tank to increase the octane rating of your fuel. There's no danger of using fuel with an octane level that's too high, so don't worry about adding too much.

Your Oil Isn't Reaching Your Engine or Your Coolant Is Leaking Into Your Oil

The oil in your car is responsible for making sure all the moving components in your engine are well-lubricated and prevented from grinding against one another. Inadequate oil in your engine causes the pistons to transfer some of the force against the cylinders when they fire, causing the shockwaves responsible for the knocking noise.

If oil isn't reaching your engine, you may be able to fix the problem by changing your oil and using low-viscosity oil. This makes it easier for your oil galleys to bring oil from the oil pan to the hydraulic lifters in your engine. You can also try adding oil detergent to your oil to clear out any accumulated sludge that may be blocking the oil. If this doesn't fix your engine knocking, then you should take your car to a mechanic before you continue to drive it – low oil in the engine causes serious damage, and the problem needs to be fixed immediately.

A leaky head gasket allows coolant to leak into your oil pan and mix with the oil, which ruins its lubricating properties and quickly corrodes your engine. You can check for a coolant leak by measuring your coolant when your engine has entirely cooled off – if it's not leaking onto the ground, it's leaking into your oil. This is another serious problem that requires immediate engine service to repair, as continuing to drive with coolant in your oil will cause catastrophic engine damage.

An Engine Rod Is Slamming Against Its Rod Bearing

When your engine is knocking, the worst-case scenario is that the sound is very loud and comes from the bottom of your engine. This is usually a rod knock, caused when the rod in an engine cylinder slams into the rod bearing after the cylinder fires due to wear and tear, low oil, or misalignment. You can check the status of your rod bearings by removing your oil pan and examining them with a flashlight – you'll be able to see gouges, cracks or holes in the bearing if you have a rod knock.

This is another problem that requires the expertise of a mechanic. Driving with a rod knock will eventually result in the rod breaking off entirely, destroying your entire engine. You'll need to have your engine machined and rebuild to fix the damage caused by the knocking rod in order to fix the problem.

Overall, if you aren't able to stop your engine from making a knocking noise by yourself, then you need to take your car to a mechanic. Neglecting to service your engine when you hear it knocking can lead to the destruction of your entire engine due to low oil pressure or broken rods. You'll save money in the long run by catching and repairing engine problems before you require a total engine replacement. 

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