youd know if the exhaust wheel was gone . the car would be slow as fuc k and it would have dropped its guts when it you were boosting it somewhere.
my guess it its the rusty f**ked "heatshield" double skin what ever the f**k you call it that surrounds the front pipe before the cat . it rusts out and the insulation eventually f**ks off and you get rattles when your engine is idling at a certian frequency and the shit stops rattling above idle
what is your front pipe like?