06 mercury grand marquis air going out the defrost and floor eatc repaired no help

I have a 06 Mercury Grand marquis that would put most air out the defrost and floor and sometimes it worked and would blow out the vents so I put new o rings in the eatc and now no air out the vents a just out the defrost and floor so I took out the eatc and I had a ball pump so I inserted the needle in the black line and pressed air through it to see if I could find a leak under the dash and could only hear air hissing under the hood next to the check valve Should you be able to hear air like that ? is it showing there is a leak there some where ? and I cant get to the check valve under the wiper box can I remove the metal wiper box without having trouble getting it back ?