Catalytic converter going out possibly? They tend to rattle after the honey comb falls apart. I would get under the car and see if there’s any parts that are touching or almost touching that could be a culprit. I don’t think it’s the mounts unless you got a defective one that already collapsed.

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1993 e300
1995 e320
1994 e320
2006 s500 4matic
2004 Jeep wj overland
2001 Ducati 748
2004 Honda shadow aero