When taking a 2004 Volvo S60 for a quick test drive to possibly purchase, I noticed a horrific howling noise when turning the wheel to a certain amount. After pulling over, I noticed that the power steering fluid looked low and somewhat bubbly. Is this what was causing this issue or is there some other underlying issue?

 

The low fluid will be what is causing the noise as the pump is starving for fluid. Where ever the fluid is leaking from is the underlying issue.

Low fluid will destroy the pump sooner than later, if not already.