rear brakes make a knocking noise

What is your question?

My rear brake shoes/pads and drums were replaced in August 2010 and when I stop at a light or brake the knocking noise appears. I took my car back to Tire Kingdom and they looked at the rear brakes and replace them with another new pairs of brake shoes/pads. Well, the knocking noise appeared again after a day. Do not know why the new brake pads/shoes and drums are making a knocking noise. Any advice will be greatly appreciated. Thanks!