The same buzz thing happened to my Bon Cajun on one of the bass reeds on the treble side when I played hard. It took a long time until i found the cause of the buzz:
It was the sealing gasket on the frame between the treble box and the bellows box that stuck out a few millimeters on the inside. When i played hard the reed or the reed leather, being very close to the frame, hit the frame gasket. I cut the bit sticking out and the problem was solved.
Another thing that may cause buzzing is very small chunks of wood or other material, left in the box or sucked in when playing. That happended to me in one of my boxes. You can use a strong but thin piece of paper and VERY gently put it under the reedtongue and move the paper back and forth a few times. Be careful not to get the paper stuck though.
The obstructive material may also be in the reed chamber. Try to open the flapper and shake the boz gently, holes down and see if something comes out, like when you try to get the plectrum out from the inside of your guitar :-)
I hope you get some help from my experiences with this wonderful instrument and please make allowance for my language.
I too had this problem with my "Bon Cajun" accordion. The buzz/rattle was on the bass side, top button, on the push. I sent it back to Larry Miller before he retired so that it could be repaired. It was eventually returned to me with a note stating that they could find no buzzing sound. I then removed it from the case and played it. The buzz/rattle was still there. So frustrating.
I just tried the advice of the earlier poster. I took it apart and noticed that one of the leather pieces wasn't flat like the others. It wasn't flush against the metal portion where the reeds are located. I carefully (it was glued) pulled that piece of leather off. Then I wet the leather, set it on my kitchen table, and pressed a flat piece of wood against it very hard for a while. I did this until the leather looked flat again. I then glued the leather back into place. Voila! No more buzz/rattle! Thank God! It was driving me crazy, lol. It's just a shame that I wasted my time & money in shipping it back to be repaired.