What is my car trying to tell me?

While driving, my car recently started to make a groaning type of noise when I turn the wheel. As the car warms up, the sound is less prominent and becomes more like a moan (but is still there). There are no idiot lights popping on, so I am unsure if it just needs power steering fluid or if it is a sign of something major. It’s a 2005 Ford Freestar and was last serviced in July. It is a bit over due, but has gone longer between services with no ill effect.
If it is the power steering fluid, does the car need to be running or the engine warmed before I add it?