97 Taurus SES light

You may now have an EGR system fault code(s). The low restriction cats sometimes result in insufficient EGR flow. Sometimes, resetting the ECM is all that is necessary. If you have a sensor near it's adaptive limit, you may have to replace it. The system is dependant upon a certain amount of back pressure to function properly. Your ECM may have simply noticed a major change in operating conditions. Now, did you check the heat shields on the cats? Many times the rattles are the heat shields that need a spot of weld to reattach them. Any good muffler shop can do this in 5 minutes or less but, it makes a lot less money. Many converters have been wrongly replaced for this reason.

Good luck Lugnut

Thanks for the tips. The "rear" cat (to the rear 3 plugs) was toast though, if you gave it a shake it sounded like someone had tossed about 5 lbs of rocks in there, just everything was breaking apart. I think what may have happend was because of the previous owners neglet. When I bought the car (for a song) about a year and a half ago, the front (easy) plugs had been changed out, but the rears were either original or at the very least exceptionally old, and the car had a miss and SES light. Changing the plugs and wires got rid of the miss nicely (and the SES) however god knows how long unburnt hydrocarbons were being shot into the rear cat.