I hope Candace will answer to your question, because she gave me very good advice when I was working on my Bernina 707.
It could be the belt that is sitting between the wheel and the motor; you would have to remove the wheel and open the side of the sewing machine to see this. But somebody told me also to be careful with that because this part is easy to distord.
Well, I'm not very helpful. Hope somebody else will answer. But as soon as there is nothing wrong with the motor, it sould be easily mendable.