Power Steering GROOAAAANNN (W126)

[JustinMorrison]

Okay, here goes...
Every other day (or so) when I start my car in the morning the power steering will groan (usually for about the first five minutes of driving). It is louder when turning left than right, though I'm not sure if that means anything. The fluid and filter were changed about five months ago, so that cannot be the problem. This started about 3 weeks ago, and nothing has happened around that area to cause this, that I can thnk of. I checked the fluid level, and I cannot see a mark of where it should be. The fluid is about half way to the top of the reservoir. Is this the correct level? And if so, what could be causing this noise? Thanks for any help!
Justin
(oh, sorry...it's a 1987 560SEL)