I'd have it checked. It might be a fault with the self-levelling rear suspension or, a couple of years ago, a reader complained of stiff suspension of a 3-Series. I told her to get it back to the dealer and, red faces all round, they discovered they had not removed the transit blocks from the suspension springs. When I drove the new F30 3-Series a few months ago the cars were all fitted with 225/45 R18 runflats (deeper sidewalls than 245/45 R18s) and rode very well.