Noisy Box/diff??? Only at slow speeds?

[Windowlicker]

Hi there,
This may be the norm, i dont know as i have had no experience of the BMW SMG boxes before.
Basically, having driven home from work, she gets a bit of a thrapping on the open roads, i pull into the cul de sac where i live, having changed into 2nd, then i slow right down to crawling pace to ease up the small stepped drive entrance, ( please note , i have not selected nor has it auto selected1st gear, as i have not come to a complete standstill) whilst doing this, and creeping up the drive entrance there are various dull clunky / thuddy (new word ) noises.
Not metallic clunks, just damped clunkings.
Only seems to do it after a proper blat, then coming to crawling pace speeds, as in parking up etc.
Is this noise just the gearbox sorting itself out and nothing to worry about , or ....are the boxes generally silent in function unless there is a problem ???
Having seen so many adds for cars when looking for mine , stating they have had new clutches and rear diffs at only 20 odd thousand miles, just made me wonder if these are fairly "Normal " replacements and time periods for things like diffs???Which sounds a bit toss to me, clutches maybe but diffs i would have expected much longer service from.
Many thanks for any replys.

Cheers, Nick