Not saying it is an exhaust leak or something else. But I would try to make some effort to narrow it down before tearing it all apart. You might be trying to fix something that isn't broke. Again not saying it's an exhaust leak but if it's the same noise your 318 made and you are using the same manifolds it might be worth checking it out and eliminating that as a possibility first. And while you're listening around you may gain some insight as to where the noise is coming from.[ Edit] You might at the least be able to determine if the noise is coming from the top or bottom of the engine.
