hissing noise occurs when vehicle goes above 35 mph. goes away when you slow down, completely away when you get below 30-35 mph.
hissing noise on 2006 Mitsubishi Endeavor
It is hard to say with out being able to hear the noise first hand, or be able to drive the vehicle and experience the problem and carry out some basic tests. If the hissing sound is when intake manifold vacuum is high (at cruise of on deceleration) it could be a vacuum leak. Could the noise you hear be a wheel bearing that is going bad. A bad wheel bearing may growl at higher speed or when the wheel that has the bad wheel bearing is "loaded" (under pressure in a turn) at speed.
check wheel bearings