2004 Ford Taurus Sounds like a small exhaust leak when firs

2004 Taurus 3.0 FlexFuel engine. When first starting the car in the morning or after it has been sitting a few hours it sounds like a small exhaust leak but after about 1 minute it seems to go away. Car runs great and gets 22mpg. Every since hearing this exhaust noise sometimes the check engine light comes on and the code is P0174/P0171. Lean condition on bank 1/2 which makes sense if I have a exhaust leak. I've tried stuffing the tailpipe to see if I could locate the leak, but I could not locate it. The check engine light never comes on when the car is running only sometimes when restarting the car. I've checked the long term fuel trim and also short term trim too and they are normal when the car is running. It appears that this only occurs at startup. Most emission parts have been replaced except for the cats. Car runs too good for a vaccum leak and I've checked all hoses, intake manifold, EGR, etc. Sparkplugs are the correct model for FlexFuel and the cars idles fine. I used a hose to try and localize the exhaust leak, but there is a heat sheild on the passenger side of the car that prevents an exact location. Have you guys heard of a problem like this before? Most supposed Ford experts keep telling me its a vacuum leak, but I've been unable to find one either in the intake or throttle body or hoses. Let me know what you think. It would be alot of work to pull the exhaust manifolds to try and find the crack or loose hardware.