VW 98 Jetta Rattling Noise

I have a '98 VW Jetta GL, 69,000 miles. It's an automatic and there has been a rattling sound when I start the car and am idling and when I am accelerating. It's in the rear of the car, on both sides. When I am accelerating, it is almost a grinding, rattling noise, and it sounds awful. I want to know what it could be and how expensive it is to repair or how serious it is!