Nixy what year is Markies car? There is a distinct difference in sound in a 276bhp engine and the 296bhp engine (deeper howl) on the 296 engine.There is also a difference between a 296bhp engine and the 309bhp HR engine
Other than that i dont know unless they have different cats on, i am assuming you are talking about cars with the standard exhaust fitted?
Did i read some where about a difference in the diameter of the exhaust pipe or did i imagine that?