If it's audible, it's most likely not something small like a vacuum line. Something like that won't really be audible from inside the car.
Especially since most of our cars have exhausts air filter mods etc etc.
I'd put my money on one of the gaskets (they're loud as). maybe turbo to manifold gasket, manifold gasket, front pipe gasket, dump gasket, either cat gasket etc. There are heaps of gaskets.