Problem solved! This afternoon I checked the power steering pump and noticed that it was covered with a thin coat of fluid and dirt. I felt the hose clamp connceting the hose from the reservoir to the pump and noticed it was loose, so I tightened it with a screwdriver and topped off the power steering fluid. The noise has not been back since. Thanks to those who replied for your helpful advice!

Toreador Red 96 Contour GL Zetec 4 Cylinder, ATX Alloy Wheels No Modifications "There's such a fine line between stupid and clever." -David St. Hubbins, "This is Spinal Tap"