Yes this happened to me at about 20k miles as well, drivers side which gets more use so I can hear the sidepipes.
There is a BMW window lift motor which is attached to a splined cable which is twisted through quite an angle. I believe the splines delaminate and shear off the motor assembly. It is terminal and the factory and dealer chain just say replacement (yikes the price!) of the whole assembly. I believe SiFab has made a few comments on it and may have an option?
