I'd say fuel pump as exhaust leak kinda chugs from the exhaust in a pulse as per the firing order.
If it is the fuel pump that's a normal sound. I'm only in Sheldon if you fancy coming by to hear another 350z on idle.
David
as is £6800 or £6000 as a clean standard 350z. if you have time and money is your aim, breaking is always the way to go sadly. mods dont always = more value in relation to what the bits costs/worth.
For that milltek you need the correct milltek centre pipe also is all, either that or get an exhaust shop to make you one up to suit your new rear silencer.
Ideally you should have double checked before hand.