i would tend to stick to the exhaust manifold gasket leaking but that would not be leaking once the engine is turned off, you may find instead of the air escaping the system it might be a vacume leak try getting some very soapy water and wetting all the tube connections around about where the noise is coming from if air is escaping there will be bubbles if it is a vucume leak you will hear and see the water being sucked in. i had a VL turbo that was making the noise you describe and it turned out it was head gasket related but it only did it on boost, and again not after the car was off. Another good way to find these noises is with a bit of tube that fits in your ear, (dont cram it in) and move the other end around where flanges and pipe connections are and it will work like a stethiscope. oh another thing a friend had a turbo deisel jakaroo and it was making this noise, it turned out there was a hole in the waste gate acctuator diaphram