Whistle in my water softener-What is it?

[sofasurfer]

I have a whistle coming from my water softener when it is in the recharge cycle. It is just a steady whistle coming from the top electronic portion. Its a Kenmore 80 Plus Super Salt Saver. Quiet a few years old. Not knowing anything about water softeners and having no idea what I am talking about, I would guess that a valve is not open all the way or is plugging up and the water is working harder to get through. I think it whistles on high volume water flow and does not whistle on low volume water flow. I don't know what the different cycles are.
So. is my guesstimate close? What do you think it is and can I fix it?