I have a 2012 MZ3 Hatch and my main rattle back there ended up being the latch. The dealership actually replaced the latch, which I knew wouldn't help because there was nothing wrong with the mechanism itself. To test if the latch is your culprit, take a wash cloth or a shop rag and lay it on the bottom part (the square "hook") of the latch. Then close your hatch on the rag and go for a drive. If that makes your rattle go away, then the latch is the culprit. I ended up fixing mine permanently by adjusting the hook. It's easy to get to. Just take out the foam around your spare tire, then you'll see 2 of those plastic rivets holding in the big plastic piece that covers the hook's bolts. Remove those rivets and then pull up on the plastic and it will come right out. Mine stopped rattling when I adjusted the hook all the way forward and all the way to the left.Hi All!
I have a 2012 MS3 and now at a 1000 miles I too have developed a slight rattle in the rear hatch area. From what I have read I have done the following: shock mounts (covered them with light grease and tightened them back up), tightened the rear wing, took everything out of the trunk, tightened the plastic nuts in the rear wheel wells, this all did nothing. However i did find that the passengers side rear belt tensioner was rattling, i lubed that up and it eliminated one of the rattles but I still have others I need to track down. Might be the electronic latch on the hatch, need to check that one still. If you want to check the belt tensioners while going over the bumps have someone or you grab the seat belts from the rear and hold them tight while you go over the bumps, if the rattle stops then you have your culprit.