I had the same problem with running lean code. I have a 99 F150 XL v6. I replaced the Idle Air Valve and that didn't do the trick. I kept hearing a loud hissing sound and placed my hand on every hose and listened. The last one I touched caused the hissing sound to stop. In my case it was the rubber elbow leading to the PCV. I absolutely could not feel a leak, but when I jiggled the hose back and forth the leak would stop. I finally felt the hole in the bottom portion of the rubber elbow leading to the intake. Will fix tomorrow. I hope this helps.

 

About 2-3 months ago I had the same problem and went AZ and the guy there found the leak in the elbow going to the PCV under the plinum. When I went to the dealer to get a replacement I took the elbow and just by looking at the part the guy knew what the part was, part number, and the size engine. I had and I never talked to them before. Whan asked he said it goes out alot.

 

Manlikebear has it right. I had the same problem. It's pretty common with the fords but it's a cheap fix. Also, a neat trick to find a leaky elbow is to spray break cleaner (flamable kind) on or near the culprit. If it's drawing air in at the point you spray, you will hear the motor rev up slightly. It's not dramatic but noticeable. You may have to spray the break cleaner and shake the fittings like they would shake going down the road.
Good luck.