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.

 

I have the same problem with my 04 t6 although not that frequently.
Have gone through 3 pumps and 2 fluid changes with no success.
recently I noticed there is a pipe inside the reservoir that is broken.
I am assuming the pipe directs the return fluid from the pump to the far end of the reservoir to give it enough time to cool down before it gets sucked back into the system. So over-heated fluid could be the cause of the problem.
I will replace the tank and will post if it solves the problem.

 

sorry I forgot to reply to this. Since this issue, I have had a couple more car with the same noise. All I have replaced is the reservoir, the new ones are redesigned where the pipe that you see inside goes all the way to the bottom of the reservoir.
also I have used after market ones ( swedish profesional parts) those ones started leaking again at the seam within warranty. Only go with the one from volvo.