The fluid might be low but not the only reason. If you already didn't know these trucks use the PS to boost the brakes versus vacuum on a gas motor. That said mine used to get louder the harder that I would press on the brake even standing still.
Ford replaced the box and pump anyways but I am sure a switch to Synthetic (Mobil 1 ATF) like I just did in my Excursion would have cured the noise. Some guys put filters on also.
Try searching www.thedieselstop.com
there are tons and tons of posts about the PS noise almost as many as leaf spring options.