I had exactly the same problem with my Janome 6500. It jumped and rattled so badly that I ended up putting the needle right through the case and had to replace it. My dealer was no help and kept claiming it was operator error (which it wasn't!!!). Finally after joining the Jamone 6500/6600 forum, I found out that there is a small oiling spot under the removable bobbin case. It's a small hole filled with a wick (looke like felt and it's not lint) that sometimes dries out and needs a drop of oil once in awhile when it gets noisy. Oil it when the bobbin starts to rattle and no more trouble! Nowhere in the manuel does it mention this spot and my dealer seemed to know nothing about it. My friend had a Janome Gem with the same noisy bobbin case. Same oil wick spot on her machine and a drop of oil solved her problem too. It could be a mis-shapen bobbin but I bet it's the dried out oil wick. Good luck with this.