Guys this comes up all the time. Happened to me and hundreds of e46 and e90 m3 owners. The clunking at low speeds, especially when changing momentum (ie slowing down then quick forward acceleration at low speeds) this is completely absolutely normal and simply a noisy differential sound. They can do the "thicker" diff fluid but didnt help me.
The only thing people should rule out is a loose/broken diff bolt. If those are fine than this is totally normal. I experience this every day each time I slow down in a parking lot and then a very quick stab of the pedal and release it than it will clunk
in conlclusion, rest east, get your car home and drive and enjoy!