I have the Janome 6500 and my bobbin case was doing the same thing. It was rattling, growling, thumping and jamming up with thread. I found out from the Janome 6500/6600 forum on line group (it wasn't in the manual!!!!) that there is an oil wick under the bobbin case in a little hole (it's not lint because some owners have pulled it out) that sometimes dries out and needs a drop of oil occassionaly. Does your machine have this too? That drop of oil made an instant fix. My dealer made me very angry and kept telling me that my problems were caused by "operator error" and never once oiled this spot or mentioned it even though I had my machine back to the shop three times and they kept it at least two or three weeks every time.