I had the same problem intermittently with my 1999 V70 for several winters. I finally solved it a few weeks ago. It turned out to be a loose connection on the suction hose for the power steering pump. In cold weather the hose was loose and would allow air to be sucked into the pump which would cause the fluid to foam and cavitate the pump. After the engine warmed up, the noise went away. The area around the power steering pump reservior was always damp but I did not realize that is was due to the foaming fluid escaping out the top. After tightening the clamp at the pump connection (4 turns!), the noise has completely disappeared. I hope this helps, I know how this problem drove me crazy!