My fiancé drives a 99 civic and I am not sure what is wrong. When she idles her car something rattles. I'm not sure what it is. When your driving there is no rattle but when your sitting at a red light there is a rattle. Could the cat be loose or rusted out? Or is this nothing exhaust related.

could be heat sheild. look to see if the exaust is hitting something

the heat shield is rusted off i just had to weld a guys to stop that