As the power steering fluid works hard (gets dirty) it can sometimes quieten down a bit if you flush the system or even try suctioning out the power steering reservoir and add new GM fluid.
Worth a try....
We haven't had it looked at yet; however we have been told by many that it is the PS pump. The fluid has never been changed; although we did add some a couple years back.
The sound does remind me of a super charger....however it is annoying. I will check into flushing and replacing the fluid...hopefully that will help.