Dont know what it is, but this started happening to my A3 when the girlfriend left the driver's side window open in a massive storm for 2 days! ! all interior soaked and also the door card. Amazingly, all worked fine but the window started scraping and sounding awful when going down. Never found out the real reason but i guessed it was rust or lack of lube in the hinges of the window mechanism. Didn't fail after a year and then sold the car...