1)Upgrade you car audio system. Remove the stock car head unit, speakers and replace them with aftermarket car audio. Make sure you use speakers that have a high rms wattage, as the more higher the wattage is, the more louder they will sound. Once you have upgraded your car's speakers, buy a ported subwoofer enclosure, tuned to 50Hz and install it into the car's trunk. Then wire the car subwoofer and speakers, using a car amplifier. If you install loud car audio equipment into your car, you will sure turn heads guaranteed!
2)If your car engine has a turbo charger, you can add a blow/dump valve and make a psshh noise. That noise will happen every time when you remove your foot of the foot pedal, or when you change gears. It sounds really cool, and make you car sound faster, as well as turning heads.
3)Add a cold air intake, into your car's engine. Adding a cold air intake, will make you car sound really aggressive and loud because it allows more air to get into the engine. It is a affordable upgrade, which also help towards making your car sound louder and help increase it's performance.
4)If your car has a turbo charger, increase the boost pressure (psi) to a higher amount. The more higher it is set, the more louder your car turbo charger inside the car engine will sound because more pressure, means more turbo spool can be heard, when you place your foot on the car throttle to its maximum level.
5)Remove the catalyst converter and replace it with a straight pipe. This will make your stock car exhaust sound louder, but on the downside, the car will not pass it's MOT and you might get a reduction of mpg from your car.
6)Drill holes in the exhaust muffler. Your car might sound loud when you do it, but holes in the exhaust increase the chances of rust getting into the exhaust. which can decrease it's lifespan.