Does it do it when you are coming to a stop at slow speeds or accelerating from slow speeds? Or when you are coasting at slow speeds and then take your foot off the gas and coast? Mine has been doing that where it sounds like a thump thump or like a loose piece sliding back and forth as I stop and go.
Full out driving there is no issue. Checked my diff bolts and one was a tad loose so tightened it but did not help. Changed my diff fluid to the one with the additive but still have it.
I am in no rush but its kind of annoying. I just turn on my stereo always and it drives perfectly so I do not care. I think noisy diffs are common in all the M cars and it just varies from car to car how noisy it is and how in tune with the sound the driver is.