1) make sure spare tire and everything in trunk is secured


if rain is a factor (as opposed to a coincidence) check to make sure rear tires are the same make and style with similar tread pattern and depth along with air pressure - I'm guessing the traction control may be going off - indicator on dash going off when noise occurs ? - will it happen if ESP button on dash is turned off?

Did I read you have new catalytic converters? if so I would guess most likely source is a missed hanger on the exhaust system, have you tried shaking the piped at different ponts to feel if your getting up-down or excessive sideways movement? maybe the rain is making things easier to slide about