2004 Buick Lesabre Brake Noise

I just had my brakes repaired about a month ago. I am getting a scrapping noise. Not everytime, but more and more frequent. My mechanic changed the disc pads twice, cleaned the dust away, and another mechanic tightened the bolts because he said they weren't 'seated' right from the previous mechanic. It sounds like its coming from the rear brakes. Both the front and rear brakes are disc. I changed the front breaks in Dec 2009 with no problems. I changed the rotors and discs when they were repaired. This has been ongoing for about a month now. They do something to fix them, but then the sound comes right back. I'll bet I've put over $300. In repair of this problem over the last month with no lasting results. The mechanics don't seem to know anything. What could be the problem? Thank you.