Cats on top on n63. PS shaking suggests a problem with the serpentine drive.

Take a slow motion video of the drive. Probably will have to take the fan out to do that. I've seen about one a year of harmonic balancer failure on the forum. Similar failure rate on the tensioner. Either can cause a shake that will rattle anything loose or even a stone sitting on the stiffening plate.

That of course doesn't explain sulfur smell but unless you also have an exhaust leak neither dou the cats. Sulfur from cats you'll smell behind the car. If you do have exhaust leak it can sound like pop or rattle. You should have some telltale soot likely just below the cat on the right side.

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2011 E70 • N55 (me)
2012 E70 • N63 (wife)