vacume leek , all hoses are good, where should i look next?

Ford is good, at hiding the parts that go bad. It is probably a main vacuum elbow, on the back right side or under the air plenium. There is no easy way to find the problem but it is easy to fix when found. run you hand around the area you think it is coming from, and listen for a change in the sound. you hear that and you are close, do a visual check of all rubber connectors on the hard plastic vacuum lines. If all fails get someone to do a smoke test for you it will show up in a few minutes.

mine does the same ,and found that the PVC VALVE is fluttering(replace it )on passenger side