Power steering

I have a 1999 Ford Taurus with an OHC engine. Two days ago my PS pump started squealing. I checked the fluid level, and it was low. So, I added more fluid according to my shop manual. The squealing stopped. Everything was normal. An hour of driving later the squeal was back. The fluid level appeared to be low again. I added more fluid, and the squeal stopped. There does not appear to be any fluid leaking under the car. I do not see any pooling of fluid anywhere. Where could the fluid be going? Could the PS filter be plugged. I'm at a lose.