PS whine??

I have had 2 excursions into some pretty gnarly mud this winter, and both times after about 30 minutes in the mud my steering starts whining. when you turn the wheel it kinda makes a sighing/whining sound that is hard to describe. Then it continues to do it for the rest of the day even after washing and then the next day its gone....very strange. what could cause this? Is it a sign of trouble to come? If so, how do I correct it?

Doug