My Janome 6500 (yours probably does the same) thunks when the small oil wick under the bobbin case dries out. Add a drop of oil in there and the thunk disappears. No where in the manual does it tell you to oil it there. I also had my machine back to the dealer several times for service and nothing was ever said about this. I finally found out about it from reading on the Janome 6500/6600 forum which was a wealth of information. Don't try to pull that wick out (it's not lint) just drop in a little oil. If you take out your bobbin case, clean under there and then add the oil, I bet your thunk will disappear.