I had the same problem and it was driving me crazy. For a long time I was not able to figure out what it was. Finally I started being suspicious about the upper boot lid. If you open it, you can see black rubber stoppers, one on the left and one on the right side. I pulled them off and fitted 1mm metal washers inside them. It lifted the rubbers up a bit and mysterious banging was gone after that. Probably buying new rubbers would have helped as well.