Whiring power steering pump

Power steering pump on my 2010 Freelander 2 TD4 was whiring this morning
Fluid was on min so topped it up and steered lock to lock and the whiring stopped
Is this fixed do you think ?
Shouldn't be whiring even on min level (engine off) should it ?

Wasn't doing it yesterday so could it be a blip, air lock, or is pump on way out ?

Thanks

My guess is the filter is beginning to clogg. I'd replace the reservoir and give it a fluid change with correct high quality fluid.