Our power steering started leaking bad two months ago. Our mechanic replaced the PS unit 3 times, but each time we get a low whining noise while steering. This last replacement the noise is lower but still there. The noise also happens when driving straight, and the sound varies with the car speed. Our mechanic is great & honest, and has solved problems in the past that specialists couldn’t solve, but they can’t seem to eliminate the noise. We had the belts replaced last December.
This is a high pressure hydraulic system. It must be devoid of air in the lines to prevent foaming of the hydraulic fluid and hose failure. After three failed attempts, I think this mechanic owes you a refund and a reference to a qualified specialist.