OK, I just had the same problem. Check the heat sheilds on your cats. If they come loose they rattle like all hell. I had to pull one of them off because it made my car sound like a POS.
Scott
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1992 GT 5.0L Convertible
BBK Shorties, Flowmasters, Super Chip, Mass Air, Under Drive Pullies, Hurst Shifter, Duck Tape
AIM: SmokeyDP