Could be the exhaust - the cats sometime start disintegrating which results in chunks of the porcelain bouncing around in the can. If you can get the car on a lift, I'd bang on the cats a little to see what it sounds like in there. Obviously, don't hit them hard enough to break them.

Here's a simple question - what kind of car is it? What year?? If it's an older diesel then the above suggestion is wrong.

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There is a very fine line between "hobby" and "mental illness."
FrankenBenz: '87 560SEC with an '02 cammed LS6 and custom 4L60E.
Bling: '87 560SEC, ported/polished, lowered
Lisa: '87 560SEL, lowered