The infamous Subie rumble that is characteristic of all WRX and WRX STi models comes from a combination of the boxer engine design and unequal-length headers. All boxer motors have slight rumble, due to their configuration. The WRX and WRX STi models really emphasize it with unequal-length headers – making for the famous Subie rumble. The unique sound comes from the uneven pace of exhaust pulses reaching the turbo. Unequal-length headers are worse for performance than equal-length headers, but they do give it an interesting sound.