Hissing sound.

    Just purchased a 2005 Sport Trac. On way home, noticed an intermittant "hissing" sound from somewhere around driver-side firewall. It isn't very loud and only noticed it when I turned the radio off. Doesn't appear to be affecting driving nor am I noticing any leaking of fluids or otherwise. Is this normal? What is causing it?

Could it be the normal noise from the plastic intake on this vehicle.  Now that it is no longer insulated by metal, air does make a noise as it is sucked into the cylinders.  I chased this when ford first came out with plastic intakes back in the 90"s.  Figured out what it was.