In addition to the above comments, try Bon Ami. BMW had a service bulletin a few years ago recommending its use, if all else fails:
BMW Service Bulletin
Bulletin Number: 61 02 94 (3990)
Noisy Wipers, Poor Cleaning: Group 61 Electrical
SUBJECT:
Windshield Wipers Are Noisy Or Do Not Clean Properly
MODELS:
All
Situation:
Windshield wiper blades may start to make noise, or fail to clean the windshield properly, even though the blades are not worn and are properly adjusted.
This can be caused by contaminants on the glass, such as oily road deposits or wax residues from car washes. These contaminants can cause the blades to chatter across the glass or fail to wipe cleanly.
Correction:
Common contaminants that are not removed by conventional glass cleaner can be cleaned with Bon Ami(R), a household cleaning product.
The cleaner should be applied on a wet cloth or sponge and the windshield must be thoroughly and evenly scrubbed. Wipe off residue with a clean,dry cloth before the product dries on the glass.
Warranty Status:
Information only.