oh welcome back to the sewing forum too!

Thank you so much! And no, I didn't know about "bobbin hygiene" but I will be sure to start practicing it. I should have known to look up the Sears or Dritz sites...

The bobbins I bought before are rounded rather than flat, so I figured that was why they rattled.

that's probably the diff. between class 66 & 15?

I dunno, I can't quite tell through the plastic. The packaging on both of them does say they're for both drop-in and front-load, but whichever. Now to find the receipt for the class 66...