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?

 

Could be the AC lines leaking as well

 

I don't recall ever hearing about tapping on anything on the driver side...?
The clown in me wants to say look for a snake, BUT...

Yes that could be a vacuum leak, 4x4 or other. Check the lines that go onto the IWE. An AC leak would not last long before it was empty/done. In order to check for 4x4 vac leaks, read through this;

http://www.ford-trucks.com/forums/836438-everything-you-wanted-to-know-about-the-iwe-system-and-then-some.html
You can check any visible vauum lines and fittings by sight/feel - if you can hear the hissing you will most likely be able to feel the airflow as well.

 

tap the transfercase motor it worked for me when it was locked up and it didnt cost me a dime.

 

Where is this located?

 

Shift motor is on the transfer case. Look in here;

http://s756.photobucket.com/albums/xx203/60DRB/06truck/

That doesn't have anything to do with any hissing noise though. When you say your 4x4 isn't working, is the 4x4 light coming on? If not, the "tap on the shift motor" is worth a shot.

 

IWE or line to it leaking

 

What's IWE?

 

Integrated wheel end (IWE) There's a solenoid on your fire wall that controls the air pressure to the IWE's ...sound's to me like it's leaking or a line running to it is leaking. You truck's 4WD system use's this air pressure to keep you truck in 2WD...they get old and start leaking or "hissing like a snake"...good luck

 

Thanks for the info guys. Trucks going in today to be looked at.