I had a few A Classes sent to me with the same problem. Basically it’s the brass counter balance where it pushes onto the motor drive shaft. The drive shaft is spinning inside the plastic housing which is incased within the brass counter balance.
It’s a very simple fix. Slide the counter balance a couple of millimetres of the drive shaft put a little dab of superglue on the shaft and push the counter balance back on and that’s it job done.
Hope this was of some help.