I don't mean to be rude but google gave me this: http://www.instructables.com/id/Make-A-Digital-Clock-From-Scratch/ Is it as straightforward as this? Sorry if I've missed something obvious
They are Helmholtz Resonators which help to give the exhaust it's deep non raspy note, google Helmholtz Resonance for a scientific explanation, I couldn't care less but Mr Hemholtz is my hero