I don’t know if anyone else has had any luck with this problem, but I for one did put a piece of pipe between the blower and the hearth which seems to help somewhat and is non combustible and so I assume safe.
I don’t know of much else that can be easily done but I personally consider it almost a flaw in the design because after playing with it on many occasions, I can’t totally stop the annoying rattle.
I haven’t tried bending the small tabs that hold the blower to the insert at this point, but my assumption is that this might be the only real solution. I don’t feel that I wish to put “non-combustible” wedges under it as my feeling is that even these could occasionally shift or move and the rattle would return.
I actually have two friends that have a Jotul “Kennebec” and have the same problem. Jotul is not one of the less expensive models so it is quite surprising they haven’t addressed this problem as it does appear that it isn’t something that is all that uncommon.
All I can say at this point is that if you figure out a “final” solution, I would love to hear about it so I can share with others.