Thanks for the info guys. It just seems to do it an awful lot though - 3 or 4 times in the last week. I never noticed my old 335d coupe doing it at all (more or less the same engine)
Last: 330cd Sport - Silver Grey, 335d M Sport Coupe, F10 535d M Sport.
Current: AW F21 M135i, Coral Red, Pro Nav, Vis, HK, & Clunky Adaptive Suspension
Current: F15 X5 XDrive 40d M Sport - Space Grey, lots of toys.