sounds to me like a wheel problem. have them all checked out-IE balanced on a machine, maybe weights have come off wheel or wheel has been damaged by a curb impact?
if this does not cure this then you need to start looking into the transmission side of things, IE the drive shafts may have been in correctly reinstalled or a driveshaft bearing is worn (middle of car underneath ) . try some basic checks/ maintenance using the BENTLY BMW downloadable pdf manual.
howtomendit.com for free repair help, information and advice.
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