The metal 66 bobbins you buy today are all from china and are of poor quality. If you buy 20 you will have to
throw 5 away. Their dimensions are not consistent. Some will be too tight in the bobbin case. You need to test
each one first in the bobbin case to see they spin free and throw any tight ones away. The plastic bobbins seem
to work much better than these new metal ones.
Also you can get a burr on the needle plate where the thread passes through and this will cause you fits. If the
needles have hit the plate by the hole their will be rough spots that the thread will catch on. Your machine should
have come with a straight stitch plate also. Put that plate on with the straight stitch foot and see if it sews better
and quieter. If so order a new zigzag plate. You can get new ones.