I bought brand new wipers that are 21", two inches bigger than the factory size. They chatter like crazy.

Could it be that a larger wiper doesn't conform to the windshield as well as the smaller factory size, thus causing the chatter??

 

It's got more to do with the pressuse thats put on the glass Vs how much surface you are covering. Back in the day BMW had a service action on wipers. If you pull up the wiper arm you will see 1 or 2 springs under the arm pulling it down to the glass. The service action was for people that complained about new wipers that chattered. the fix was going from one spring to two springs under the arm to increase thr pressure.

 

I already bought new arms ($70)...nice and tight...but still chatter city!?