well, it is not the winscreen itself. i ahve placed V Cool on all the screens.  the sound comes when the wiper hits both the ends during wiping, and the sound does not come from the screen but at the edge ... as the wiper moves to the left or the right edge, it is like hitting something, and it goes 'duk' and 'duk'.  yes, i do use screen treatment too., but still cannot.

how?

thank you.