If it's well lubricated, and has been sucessfully initilized with the problem still occurring... It will need a new window motor. I've seen this several times on 1ers..
'11 BSM 135i DCT - BMS DP, BMS intake, PE res delete, VRSF 7", CP-E CP, JB4 ISO 5_3, 60% E85, Inline 255, PureTurbos Stg1, CCW Classic.
'97 CSM M3/4/5 - GT3582RDBB, TRM IC, Tial 38, Synapse BOV, RallyRoad 300 FP, 60#, TRM tune, Clutchnet st5, Kosei K1. The endless project.
'98 JB Z3 2.8 Roadster - stock. 206k and climbing.