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Might be the PS reservoir... that contains a built-in filter so the reservoir should be replaced periodically. I do it when I change the fluid every 40k/five years. If the filter is clogged, it might cause pump cavitation.
Probably should bleed it thoroughly to make sure all of the bubbles are out too.
Replace res and refill first.
Then with the front wheels jacked up start by turning the steering wheel back and forth stop-to-stop multiple times with the engine off. Then repeat with engine on. You can also do it with wheels on the ground, just going to need arm strength with the engine off.