It is weird when the fan does kick in - there doesn't seem to be anything specific to cause it. The noise is not only worrying but embarrassing with it!
The last-but-one time when mine done it I didn't even turn the engine off and went straight to Benhams in Wolverhampton...***'s law, it stopped by the time I got there
But their service guy (who I wasn't keen on to be honest) said it was normal and a common 'complaint' with the Cooper D.
The last time mine done it it carried on for well over 5 minutes after I'd turned the engine off and that was a worry.
I wonder what causes it?