I have a customers car that had powersteering noise when it heated up.
I checked the fluid first and it was good, then I had it flushed with the proper fluid for the volvos. It seemed to help for a month or so. Then the noise came back. I ordered a used pump and the noise went away. a month later it came back, so i tried another pump that I had at my shop that was leaking, but i wanted to see if the noise would go away. It did not.

I have not seen this issue before on these cars. whenever i hear the whinning noise and the fluid is full, a flush always takes car of the noise.

I suspect maybe blockage in the rack, but i dont want to change a rack on a hunch.