An affordable and simple after-market modification is an intake. The term "intake" is used loosely as this could refer to a larger intake, a thicker filter, a thinner filter or a cold air intake. All of these affect the sound of the engine.
There are a few different options available for modification to your intake. A larger intake provides more air to the engine. A thicker filter gives the engine better protection but increases the resistance of the air. A thinner filter leaves the engine more susceptible to particulates but decreases the resistance of the air to the engine. And lastly, a cold air intake provides insulation to the filter to prevent the air from being preheated to the engine. This allows more air to be blown into the combustion chamber as cold air is more dense and therefore takes up less volume.
There are two areas where there will be noticeable sound difference; the exhaust and the engine. In each scenario the engine will usually work harder. This is either a result of increased combustion due to a higher volume of air delivered or it's a result of more air restriction to the engine. Either way, the engine will work harder and therefore produce more noise. The exhaust will only see an increase in sound if there is an increase in combustion. This means that all the after-market options increase the sound of the exhaust except increased air filter thickness.
All of the scenarios discussed assume that an increase in air has been accompanied by an increase in fuel. Without matching fuel, increased air will not increase power.
