I have a '98 S-500 which is typically very quiet although I have noticed that there is now a slight amount of wind noise from the driver's side door/window which seems to be coming from the upper corner (near the B-pillar). I've tried re-seating the window but it does not seem to make much difference.

Anything obvious that I should check?

Get some slick lubricant and put some on your fingers and rub it all over the weather stripping. Usually over time the rubber will harden and loose its ability to keep a tight seal. I use a Mercedes-Benz sunroof oil for this purpose. I think its about $55.00 or so, but the bottle should last you a little while. Only other alternative I can see without road testing and diagnosing the car would be to replace the weather stripping.

That's odd, because I have the same problem but it's coming from the drivers side rear.

lube is good, make sure it has no petroliumn in it...I just took mine to a glass shop to fix some wind noise....they told me about the lube and they adjusted the window

What did they charge you for the work?

Do you have a part number for this, or are you talking about the same stuff you would use to lube the rails for the sunroof?
Bruce