Thank you for this thread! Years ago, dear friend found me a 99k in a cabinet through freecycle. It had been used as an aquarium stand for as long as they had it. Nearly pristine on the inside, it was so noisy I wouldn't use it beyond piecing a quilt top for that same friend.
I now have a sewing room and lifted it out to use today. Still NOISY bobbin nearly had me put it back in. So thankful for your explanation and background sleuthing. It continues to be louder than my 201K but much better than before. You've saved it from the closet!