High-rpm engines with multiple valves and turbochargers tend to make the best intake noises.
A cold-air intake -- one ducted to bring air in from outside the engine bay -- won't necessarily change the sound of a car, but a short intake will. This noise can be quite pronounced, though some manufacturers go to great lengths to reduce the engine's intake noise, going as far as to install a type of muffler in the intake tube.
Intake Noise
Intake noise comes in two basic varieties: noise that comes from the sound of air rushing into the intake, and the sound of mechanical components, such as the turbocharger blades, fuel injectors and valves opening and closing. Turbocharger blades whine like a jet engine, and air going into the engine can vary from a slight whoosh to a roar, for large engines under full throttle. While a single valve may only make a slight, metallic tap when closing, dozens of valves closing dozens of times per second can make a particular and pleasant sort of a ringing noise.
Optimizing Sound
Sound propagates through an intake system just like it does through an exhaust system; larger-diameter tubes will emit a deeper note, and shorter tubes will make it louder. Intake noises can be quite loud, particularly for turbocharged cars, or for cars using three or four valves per cylinder. While sound preference is a fairly subjective thing, intake noises are generally more pleasant than exhaust noises. If you've got an engine with multiple valves, go for a large, short intake tube -- it sounds cool and racy, without being obnoxiously loud, and it might even add some horsepower, depending upon the application.
Tags: closing dozens, intake noises, intake tube, multiple valves, short intake