Engine Noise: Hiss

WARNING: This page is about a different car, the 1996 Ford Ranger. However, it is still accessible from the selected car via links, so may be relevant.

Check for cooling system leaks.
Check for vacuum leaks.
Check for loose spark plugs.
Check for air intake leaks.
Check for secondary air injection leaks.
Check for Evaporative Emission (EVAP) system leaks.