If your vehicle is showing foamy or bubbly power steering fluid visible in the reservoir, here are the most likely causes and the parts you'll need to fix each one.
When the fluid level drops below the minimum line, the pump sucks in air along with the fluid, causing it to aerate and foam. This usually indicates a leak somewhere in the system.
If power steering components were recently replaced or the fluid was flushed and the system wasn't properly bled, trapped air will circulate through the pump and create foamy fluid.
The hose connecting the reservoir to the pump operates under suction. If the hose degrades, hardens, or its O-ring fails, it will pull air into the fluid stream without necessarily leaking fluid onto the ground.
Many reservoirs have a built-in fine mesh filter screen. If it gets clogged with debris or sludge, the pump starves for fluid and causes cavitation, which creates bubbles and a whining noise.
A worn seal on the power steering pump's main shaft can allow air to be drawn directly into the pump housing during operation, severely aerating the fluid.