A problem like that-thats hard to duplicate is a pain in the butt
But you can hear the vacuum leak?
It ges away at idle?
It could be the idle air control you are hearing.
Scanning them doesent tell you much,unless you are an old ford tech with the correct scan tool.
Those ones are a hands on- test the part kida thing.
Dont know if its possible, but i have made block-off plates to test for bad egr valves. Need a gasket and a piece of sheet metal.
