Chatter from W124 monowipers is common. On my car, it happens at the extreme ends of each swipe. I started a thread on this a while back. Very frustrating and annoying, and has led to more than a few folks selling their W124s because they got tired of it.

Sometimes cleaning the glass helps. MB windshields seem to pit more than others, and the pits hold onto oil/grease from the road. Takes multiple cleanings with lots of cleaning solution to really clean them out. Some folks use a strong solvent like wax remover (used by body shops before painting).

Something that helps mine, at least for a while, is to lightly lube the chrome wiper shaft with silicone "grease" every oil change.

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Steve
'93 400E