John,
you stated that the noise changes when you load the steering OR brakes... i'm assuming then that you have a cummins? usually on the deisel applications the steering and brakes are hydraulically assisted via the pump.

in my case it was just steering as i have the 5.7hemi.

i believe there is some history of the deisel applicaitons having a problem with a plastic "isolator" inside the HP line coming loose and causing the vibration/hissing sound. should be an easy fix and obviously covered under warranty.

 

i started by replacing a leaking power steering hose and after that got a noise so i changed the pump and still had the noise so i changed the steering box and still had the noise i am going to replace the hose one more time ma be a bad hose i hope

 

I used some lucas power steering fluid treatment which i believe slightly thickens the ps fluid, my noise reduced by about 90% and so far no increase