Engine knocking is a sound an engine makes that is often described as a metallic, hollow, hammering, or rattling sound. It is typically heard upon acceleration and is often the result of an irregular combustion process. For example, an engine's air-fuel mixture may ignite too early or spontaneously combust. In some cases, improper gas octane is the cause of engine knocking. Sometimes, however, the sounds may be caused by mechanical problems, such as defective bearings, rather than improper combustion processes.

Often, a problem with the combustion process is at the root of engine knocking issues. In some cases, this occurs when the engine's mixture of fuel and air ignite too early. The irregular explosions that result from such problems can lead to damaged pistons, damage to other parts of the engine, and premature engine wear.

Commonly, problems with an engine's air-fuel mixture cause engine knocking sounds to occur. There is an optimal mixture of air and fuel that is good for an engine. Too much air results in a lean mixture, which can cause engine knocking and damage.

In some cases, gasoline problems cause the irregular combustion that leads to knocking sounds in an engine. This can occur because of problems with gasoline octane. Gasoline's octane rating signifies the amount of compression the fuel can handle before spontaneous ignition. When this spontaneous ignition happens instead of the normal ignition, engine knocking may occur and can cause damage to the engine. The lowest octane gases are typically the least capable of withstanding compression and are more likely to ignite this way.

Engine knocking may also occur because an engine gets too hot. In such a case, the air-fuel mixture may actually ignite without any inducement. Such a spontaneous explosion is possible when the engine's cooling system fails at controlling the temperature of the combustion chamber. The spontaneous combustion that happens in such a case is often referred to as pre-ignition.

Sometimes the sound of engine knocking is caused by something other than the typical combustion process problems. For example, some types of mechanical issues may cause these knocking sounds. Such mechanical issues can include defects in the crankshaft bearings, broken flywheels, and water pump bearings that are worn out. In some cases, a loose timing belt tensioner can cause knocking sounds, as can a problem with the air conditioner compressor. Likewise, an alternator's worn rotor bearings may produce a knocking sound.