When turning left or right as soon as there is some pressure I get a loud hissing from down near the pedals. Never heard this before, any help ?
Steering pump?

 

I have this problem occur every once in a while. Its a crap shoot with this hydro boost system... You could replace the pump and lines and be fine. Or you replace the hydroboost unit and be fine. Or you have to replace everything in the hydraulic system-Pump Unit and lines. I dont have a gauge high enough to read pressures and neither does my dealership. Theyre tricky to diagnose and gets expensive quick when you throw parts at it.

 

Yea my hissing went away and now it's back with any real pressure on steering wheel. Turns fine. Sounds like hissing is coming from under steering wheel itself. Any ideas on pinpointing it?

 

my 03' 6L does the same... dealer told me its because my 35" tires and lift kit??? there are a few threads on doing a filtration of the powersteering fluid for brake fluid gets in??? i dont know the details but hopefully this helps a little

 

So you did replace the hydroboost? or no. Check for some leaks too, both on the bottom of the vehicle and in the cab by the p-brake.