Hmm. I thought these engines had a timing chain not a belt? Plus variable valve timing and variable valve lift?
I doubt it's anything very bad. BMW have put this new engine through millions of miles of testing on dyno's and fitted to test cars before it went to market. As with any engine, sometimes a 'bad' on will come off the production line. From what you've written, I would guess that it's part of the variable valve timing mechanisms which is not working properly.
BMW / MINI will fix it under warranty, and in the worst case, you get a new replacement engine fitted at no cost. I am not aware of any other reports of engine problems.