Mine has been doing that for awhile, actually.
Sometimes it does it and sometimes it doesn't, right? Mine went away for quite awhile, and it never caused a problem, nor did any of the servicing it got turn up anything wrong. My power steering seal WAS leaking a few days ago before it was replaced, but it was a small leak and the car's been making that noise for a LONG time. Not to mention that leak was most likely my fault ..
Fortunately, the noise was never frequent. :s