http://www.bimmerboard.com/forums/posts/292534

Chattering windshield wipers is not always caused by improper angle. Most of the time it is caused by worn wiping surfaces on the blade itself. One way to stop it is to replace them. Another is to by a 3M sanding pad in 320 grit and pass it along the blades' wiping surfaces a few times lightly. This will make them work smoothly again for a few more months. It always works for me.

To kepp it from happening don't use your wipers until the window is really wet. Remember water is the blade's lubricant!

Use Rain-X on your windshield regularly. You won't have to worry about windshield wipers.

Many wipers come with a small tube of lubricant (silicone, I believe) which will rejuvenate the blade after UV induced deterioration. Some people throw this away, but few use even half of it when they do use it, so keep the rest in a baggie somewhere in your car...use as necessary, replace the blades when exhausted.

Or, as some from Arizona would suggest, move to Phoenix where it never rains. Fastest growing state in the US right now.

jake