On my 2008 Roadster I had a knocking when setting of from a start and also if I pulled away in 2nd gear with my foot down hard. I looked for the problem for ages it seemed to be coming from behind the drivers seat. It turned out to be the nuts on the u bolts that go over the axle. The outside u bolt, the nuts were not tight enough which allowed some movement which caused the cracking noise.
Might be worth checking them, sorry I don't have any pictures.
