I'm starting to get that buzz sound and the raise presser foot and needle bar, turn power on again. It's done it occasionally since I've had the machine but now if I put the straight stitch plate in and turn it on, it does it every time. With the zigzag plate it does it occasionally. 
I know others have had this problem and I seem to remember a post last year or more than someone found out what was causing it. 
I need to drop it off with my tech and I want to give him as much info as I can collect.
Sandy