i have a its a 89 gt and lately there has been a noise comming from under my car, sounds like something is grinding or rattling aroung. its a pretty loud noise and it doesnt speed up with the rpms at all. i really only hear it when the rpms get up over about 1500 but once the car has warmed up i can hear at idle. the car still drives just the same. the noise is definately comming from under the car in about the middle. i got under and listened and it sounds like its comming from one of the cats or atleast in that area, maybe the shifter? will the cats make a rattling sounds if they are goin bad? if not, anyone have any ideas of what it might be? thanks