That knocking noise under your hood probably isn't the sound of opportunity; it's either the sonic manifestation of uncontrolled explosions crashing together or that of heavy metal things banging together in ways that they shouldn't be. Whether you call it knock, ping, detonation or preignition, this problem is one that you ignore at your peril.
Under normal conditions, the air and fuel in your engine's cylinder gets squeezed into the combustion chamber by a rising piston. A spark plug ignites the air-fuel mixture just before the piston reaches the top of its travel, and the burning air and fuel mixture spreads away from the spark plug in a bubble-shaped "flame front." Under some conditions, the air-fuel mixture in one part of the combustion chamber explodes independently. When the supersonic shock wave from that explosion's flame front crashes into the primary explosion's shock wave, they repeatedly bounce off of each other and the cylinder walls. This creates a metallic ping, a sharp rise in cylinder pressure and heat that can melt and punch holes in components.
Anything that increases temperature or pressure in the cylinder will increase the odds of both abnormal combustion and preignition. These temperature and pressure spikes come from using too high of a compression for the octane used, an air-fuel mixture that doesn't contain enough fuel to cool the cylinder, typically anything higher than a 15-to-1, excess ignition advance, excess turbo or supercharger boost without intercooling, lack of oil, exhaust gases trapped in the cylinder and engine overheating. Detonation under boost will kill most engines quickly, especially those that use shatter-prone hypereutectic -- high silicone content -- pistons. Don't bother listening for knock when using nitrous oxide; if detonation occurs while your engine is on the bottle, then odds are that the whole thing will explode before you have a chance to think about it.
Heat is movement or kinetic energy on a molecular level; when you compress gas molecules closer together, they're forced to bounce off of each other, speeding up in order to maintain the same amount of net energy. This increase in kinetic energy manifests as an increase in heat, which is why compressing the air causes it to heat up. A diesel engine works by compressing air until it gets hot enough to ignite the fuel without a spark. Gasoline automatically ignites at a much lower temperature than diesel, so any hot spots in the chamber may increase the likelihood of gas igniting before the spark plug goes off.
Preignition requires both a hot spot in the chamber and a steady source of fuel, particularly in cases where the engine continues to diesel or "run-on" after you shut the ignition off. The hot spot is usually either carbon deposits or spark plugs heated to glowing red by heat in the combustion chamber, and the fuel is generally either gasoline drawn through the carburetor or oil leaking into the cylinder. Leaking injectors are another possible, albeit unlikely, fuel source.
Eliminate the hot spots in your chamber by running a carbon-dissolving additive in your fuel for a few weeks or months and by swapping out your spark plugs for those in a "colder" heat range. Getting rid of the fuel source is a bit more problematic, especially if it's oil leaking into the cylinder, which is probably where your carbon deposits came from in the first place, but switching to a high-mileage synthetic oil with a "seal-conditioning" additive will help.
