Just because the fluid's full doesn't mean your PS is OK.

See whether it's the belt. Spray some water (or better: belt dressing) on the belt & see if that makes it quiet. If not, then try to pinpoint the location of the noise. Can you have someone rock the steering wheel side-to-side while you listen under the hood?

It might be as simple as a loose belt. Push against the long span of the belt, it should give about 1/4" without using excessive force.

If it IS the belt squealing but it's already properly tight, get a new belt. Old ones get hard & slippery. They can slip & squeal even when they're properly tight.