im no real expert. but i would say that IF they really dont have those upgrades, its more than likely their exhaust and they have a manual transmission. Manual transmission doesn't really make too big of a difference, but it does help make that sound by revving to what you wanna rev it to which might make the engine cause the exhaust to make that noise when you let off to shift gears.
My guess is that they really do have those upgrades. I not exactly sure. but otherwise its more than likely their exhaust. Good luck.
