had this, turned out it was a loose clip on the drivers side exhaust, roughly around the position directly under the drivers seat. Just a metal little clip that came loose and would kinddddd of rattle. Wouldnt really make the noise unless i was at a stop then as soon as i would start going it would make the noise. The tech fixed it by taking it off and "stretching it" then putting it back in place. hope that helps.
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