there is a great group for Janome 6600 owners with over 5000 members and there is a lot of great information on there. One of the answers on the bobbin rattling was to make sure the wick is oiled. There are many posts on that board that tell you how to do that. I joined two months ago and have learned so much about my machine. Here is the link
http://groups.yahoo.com/group/Janome6500/