The rear exhaust heatshields rust away around the fixing points, so becoming loose and rattly.
You need some rubber washers. I got these by contacting the eBay seller worldofwashers. They measure 29mm (outside diameter) by 8mm (inside diameter) by 3mm (thickness). Slightly different sizes should work but I think 28mm would be the minimum diameter.
Simply remove the nut from the stud (spray with PlusGas first) then reinstall it along with one of the rubber washers. There's four fixings on each heatshield. I managed to change two of them - the other two might require dropping the exhaust out of the way.