A common and difficult leak to find is where the header collector receives the pipes from the engine. I experienced this twice on the passenger side header, (i beleive they were flow tech,). It makes a leaky sound that seems like it comes from the rear of the engine because it leaks in the foreward direction( from the center of the four pipes). It is a bad weld in the center that blows exhaust gases foreward. To check, start engine cold and high idle. Feel for any air flow around the pipes on the engine side of collector (where all four pipes feed in) before it gets too hot to get close. Repeat on other header collector. If you feel any air pressure, the collector weld is bad and there is no fix but to replace the defective header. If you have a bad header, you'll be lucky if it is on the passenger side. Hope this helps, took me a while to find mine. Ended up replacing w/ hooker competition, so far so good.