I have a different setup, but for what it's worth, I just converted my '70 Challenger to a coilover front suspension with a power steering rack and a GM Type2 power steering pump. When steering back and forth in the garage my steering column had an annoying vibration along with that hissing sound. Turns out the pump was making slightly too much pressure for the Ford style rack.
I installed a power steering pressure regulator inline with the feed and return lines(the excess fluid goes right into the return line so no extra heat is added to the pump) and now my steering is silent and I can adjust the feel of the system to my liking around town and on the highway.
I have no idea if this will help you or not but it's a thought.

The regulator is Heidt's part number PS-101 and all my lines are 6an. I also ran the return line through a small fluid cooler. It was installed from the beginning, before the regulator was added.