Sometimes my car will rattle or shake slightly at idle also. I can feel it through the brake pedal, if that makes sense. It is bugging the hell out of me, even though it happens very rarely. It may not be the same problem but I hope I find out the problem. Misfire?? I don't know. Anyway, I'm changing my plugs (cheap Bosch) to AC Delco, and installing my Taylor wires tomorrow. Hopefully it won't happen again.
I also had a rattling sound from what I was sure was the Cat converter. It happened at idle and got quieter when driving. Somehow I haven't heard it in several months.