[Three_thirty_I]

That's odd. Have you had the exhaust system checked for leaks or anything of the sort? One thing that comes to mind of course is the exhaust flap, these can become sticky and by themselves can rattle, but somehow it's as if it is not closing when you are in rainy conditions. But it depends on exactly what the exhaust system sounds like. You can of course test this by unplugging the exhaust flap regulator (will store an error code though but nothing serious), but I would start by having the exhaust system inspected.

Hopefully more will post with other helpful advice!