1998 c280 80k miles, when car is in drive or reverse with foot on break, rattling occurs from under car. When put in nuetral or park it stops. Is this a sign of a bad catalytic converter? If not, any ideas would be appreciated

Could be a heat shield rattling, loose exhaust donut. You will have to get under the car and push pull, pry to see what is making the noise.

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1991 300E - 212K and rising fast...