Which type of scraper are you using?
What materials are you running across with the blade? Paint? Hardwater? Silicone? Cement? Stucco?
What soap are you using?
What type of windows? Tempered Glass? Residential Glass?
Are you hearing pitch shifts in the blade as it is drawn across the surface?
It can be a matter of angle of the blade when applying the pressure stroke. Are you picking up the blade off the glass on the return stroke? If not some debris can become lodged under the blade and drug backwards gouging the surface.
More detail can give us better idea of the situations for advice.