Your sig says that you have a bomz air intake... From what I have heard, open air intakes on duratecs are MEGA loud... Just like on my Zetec. What I have noticed about my Zetec is that my air intake is actually louder then my car's exhaust when I really get on it. But on regular driving with the window down I actually can hear the air intake suck in air, it does sound like the engine is "sipping" air in through the intake. Pop you're hood and rev the engine from the throttle body and listen to you're air intake. Im sure this is what you are hearing... If not... Are there any CEL's on?
