Fast240Z

my Z is making a strange noise. When the car starts, it makes this hiss woosh noise, almost like a vacuum leak or something like that. I traced all of my vacuum lines (like all 5 of them, lol), and none of those were making the noise. I noticed that the sound was coming from under the valve cover. What could the noise be? I'm going to take the valve cover off tomorrow and check to see what it could be, I just wanted to know what to look for.