stilton,
It might help prospective respondants if you can provide the typical Thrust setting when you notice the 'below 250 Knots' buffet. Is it during climb, cruise, holding, idle descent etc.
As a 'knee jerk' response I would say that if this buffet occurs over the entire useable thrust range that it is aerodynamic. If, on the other hand, it's during typical holding thrust settings for example, then it's an engine phenomena, not unusual in the mid-thrust range for high bypass engines.
Regards,
Old Smokey