Is the wiper arm loose on the post? Does the blade always travel the same path and always park at the same place every time when you turn the wipers off? Also if you take the arm off the post, is there some play in the post - that is, is the bushing worn? On my 850 the driver's side arm got loose repeatedly and sometimes made noise. I shimmed between the arm and the post with a little piece of waterproof aluminum oxide paper, the kind you use for wet sanding. On a previous car I had erratic wiping because the bushing in the wiper drive wore eccentrically. Cure was to replace the wiper drive (keeping the motor) $$$.