Power steering noise

I have a 2001 Town & Country LXI with a 3.8 engine. I bought it used with 45,000 miles on it. Ever since the first winter whenever the temperature goes below 45 the power steering pump makes a high pitched groaning noise. The noise lasts anywhere from a couple of seconds to 20 or so seconds depending on the temperature. I change the fluid slowly over a few days but doing that had no effect. Any ideas???



Thanks,

Lou