3.3L P/S makes Growling Noise after Engine Warmup

[swimmer]

My '91 GC 3.3L steers fine and without any noise for the first ten to fifteen minutes after engine startup. Then, after engine warmup, when I turn the wheel at low speeds it makes a growling sound and is slightly tougher to turn wheel.

Hoses look fine. Fluid color and level look fine. No fluid leaks. P/S hoses not rubbing and are mounted properly. Serpentine belt new and tension seems fine. New tensioner installed. [Mechanic replaced pump less than 1000 miles ago but still no improvement.]

Could this be steering rack problem? Clogged resevoir/hose problem? The heated condition seems to be at the core of this issue. Thanks in advance for any help.