The short answer is that the wipers are likely worn out. Windshield wipers are in a harsh environment; ice, snow, rain, and then baked in the sun. The rubber will deteriorate and generally last for 6-18 months. The biggest contributor to wiper blades "chattering" is dirt and debris on the windshield. To make your wiper blades last longer make sure to regularly clean the glass and not to let the wiper blades run on a dry windshield. Clean the entire windshield with glass cleaner and a towel. You should also clean the wiping edge of the blade.
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