A year ago I loaned my Brother XL-2600i to my kid's school, and only now have I tried using it again. Yep: they messed it up. The thread tangles up around the bobbin, and it makes grinding noises.
Disassembling it, I found a broken needle inside, under the bobbin case. I removed that, cleaned it all out, and still it isn't functioning smoothly. So, I'm guessing something got bent when the needle whacked into it and broke.
Trying to figure out what got bent, I disassembled everything, trying to find out what isn't working smoothly. When I removed the bobbin case, I revealed the drum with the hook that holds the bobbin, and turns under it (the black hook is visible on the right under the white plastic piece):
Now the drum spins smoothly, and there's no more grinding noises (not surprising, as there's not a lot there). However, if I replace the bobbin case, when I run the sewing machine the bobbin case rocks back and forth (moving about ½ mm each way):
It looks as if when the hook moves under the bobbin case it pushes the case back and forth.
Here's the question: is this normal motion of the bobbin case? Or should the bobbin case be truly motionless, and this shows that my problem is with the case and/or the underlying drum?
(I'd also be happy with any other suggestions of what might be going wrong.)
Edit: I'm adding some more information that led to the solution. When malfunctioning, the loops of thread under the cloth being sewn that should have been pulled tight, weren't. Each time I tried sewing I had to pull a wad of these loops out of the mechanism under the sewing platform. Here's a picture of the loops (showing the underside of the cloth):