Could be any number of things. First off, you have to check the fluid level in the pump. If it's low, you'll have to do two things: one, fill it and two, find out where in the system the leak is.
If the fluid level is okay, then further diagnosis would be required.
Here's a Video that explains how the power steering system works and what might be wrong with yours.
Depending on the noise, there is a filter in the reservoir that gets restricted and you have to replace the reservoir to service it. Another common issue has to do with cavitation in the return line. The fix is to lengthen the hose (to 36") in the return line as per a TSB. Either of these conditions need to be confirmed before simply trying them to see if that corrects the noise this time.
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If the fluid level is okay, then further diagnosis would be required.
Here's a Video that explains how the power steering system works and what might be wrong with yours.