2006 Saturn Ion M Driving Come Complete Stop Sign Red Light Rpms Drop 700

When I'm driving,when I come to a complete stop at a stop sign or a red light, the rpms drop from 700 to 500. Then it jumps back up to 700. This happens once again. Also when this happens, sometimes when it hits 500 rpms there is a rattling noise. Goes away when the rpms goes back to 700. But on rare occasions the rpms does not go back to 700. I'm thinking it could be the alternator. The lights do dim. Could it be the alternator?