My 4x4 wasn't working again so I opened up the hood to tap the plastic black box that is on the drivers side above the wheel as some of you said to do. The 4x4 didn't work by doing that.

While I was under there I heard a constant hissing sound, like air coming out of a tire from the passengers side of the engine, almost up against where the firewall is. Would this be a vacuum leak sound or is this sound normal?

 

Most likely. What motor do you have? Is the check engine light on, if not it will probably be on soon.

Probably in the PCV system, 5.4's are at the back of the plenum by the fire wall, I think the 4.6's are too. 4.2's would be on the passenger side valve cover (that is where the hose connects, not the PCV location).

Could be other vac lines too.

Good luck.

 

5.4 and I have no lights on