2002 Volkswagen Rabbit

hi recently brought a volkswagon golf v5,when reversing and braking the discs make a really awful noise squeaky,grinding noise,it does not do it under normal driving condition and i have also had the brakes checked and was told there is a little bit of wear on discs but are ok,he said that where car has not been driven for a while it was a build up of deposits on disc,how long would you think it would keep making this noise for, or would you get a second opion any help most thankfull