I have a 2006 e92 2.5 auto that has a cracking noise that happens when I move forward or reverse. I thought it sounded like a heat shield/ exhaust. Definitely metal on metal and a quick bang.
I had it at a BMW specialist local to me yesterday and he reckons its the diff/prop shaft making the noise. Funny, I heard a E90 pulling away from traffic lights when I was walking today and it cracked and made the same noise.
Had a search online and there are a few theories, but I will hang off from a new diff just now..