Steering pump

Is the fluid boiling,.... or foaming. My guess is that it is foaming. Check for a leak on the return line from the steering gear, you may be pulling in air, causing it to look foamy. Keep in mind that line is not under pressure so it may not have a visible leak while running, where fluid is coming out, rather it may be pulling in air. Could leak when shut off though.

The noise you are hearing could be cavitation in the pump. If that's the case, you want to get it fixed soon. Cavitation can cause major issues inside the pump.