Odd sound from steering wheel

My 2002 Mercedes C240 makes a strange groaning sound when the steering wheel is turned, both at rest and while moving, but only when the car is first turned on (probably still cool). Has anyone had this problem, and do you know what causes it? Is it dangerous, and does it require fixing?

Have you checked the level of the power steering fluid?

Have you ever looked under the hood of a Mercedes? Everything, I mean everything is covered in locked plastic compartments. However, the owners manual assures me that if any fluid level is low, an automatic message will be displayed on the instrument panel. Nothing has come up.
Listening to this sound again, this time with my wife, who hears better than I do (or so she repeatedly tells me), it turns out this is a constant low-grade humming sound whenever the engine is turned on. It is only really loud when the car is first turned on, and becomes quieter as the engine warms up. I’m stumped.