Need Help w/ Hissing Sounds

[1991grandwoody]

Hissing sounds in my 1991 Grand Wagoneer.

Sometimes when I left off the gas pedal I get a hissing sound.

Most times when applying the brakes I get a hissing sound.

I always get a hissing sound when in 4WD.

One problem or three problems? What to look for?

Thanks for your help and suggestions.
1991 Grand Wagoneer
1972 BMW R75/5 Cafe Racer

[HOOT-N-JEEPS]

#1 Check for snakes..

#2 Sounds like a vacuum leak..Sounds like you may have a bad 4WD switch..Or see #1

[carrotman]

Not familiar with 91, but sounds like you have several leaks. I would plug every port on the carb/manifold and get the carb running right. Slap on a vacuum gauge, connect each hose and try that vacuum motor. If the gauge goes down at all, the hose or vacuum motor is leaking. If the vacuum is low with all the ports plugged, the carb or manifold may be leaking. Also check PCV and EGR valves if you have them.

[710 Burner]

Boy, you can tell the people from the western half.
Check for snakes.
I had to run outside an look...They are not using the Ford method of a vacuum line to kill the cruise control, so that leaves 4wd switch and heater control.

[710 Burner]

Oh yeah, I almost forgot. If your vacuum booster for your brakes starts leaking, more noise when brake applied. It gets louder when you take your foot off the gas pedal because the vacuum in the manifold increases to its maximum point then.

[JeepBountyHunter]

While your in it, switch from 2 wheel drive to 4, if you hear that hiss, start with the 4wd switch and the tubing from it...