Fart can is a derogatory name for a fairly standard feature of badly modified rice burners. It is a massively oversized exhaust tip (usually, large enough to contain a grapefruit, coconut, or, occasionally, bowling ball) which serves to resonate with the pulses of exhaust gases leaving the automobile's muffler in an attempt to make the car's engine sound larger and more powerful. In reality, though, it causes the exhaust note to become a sound which could be described as a cross between flatulence (I'm talking earth-shaking flatulence here) and ball bearings being shaken in a tin can. Its sound is often heard in conjunction with a ghetto blaster rattling the trunk lid, something which makes causes stopping at a traffic light to become an irritating experience.
Remember:
Friends Don't Let Friends Drive With Fart Cans.