Air Leaking Sound: Any ideas?

[TIMSPEED]

When I drive my truck, I hear a sound like air is leaking from somewhere. It's only like when I go from stop to part throttle. (Like off a stop sign). It's like a "shh shh shh shh shh shh" sound. I've checked to make sure all my header bolts were tight. The header gaskets are brand new. I don't have any smog equipment. (Such as an AIR Pump, that MIGHT make that noise) any ideas?
I got to thinking, that maybe that's just a normal sound for long tube headers to make on trucks. (Like air rushing through the primary tubes)