sorry yea ... potty mouth. musta been reading ellies posts on fb and got swept up in it. my bad
it is this baby from LMI
There is some tension on it, but not a lot. Only enough to maintain correct relief. At that tension, it appears there is a sympathetic resonance of the rod itself that kills the note. The strange thing (which I tried to get across before) is that when the instrument is played on its back, the effect of gravity must bow the rod down ever so slightly and it audibly vibrates agains something inside the channel - I assume it is the back wall - at the same note. Pretty good evidence that that rod is the culprit.
My fix would be to remove the FB, and:
* damp the rod with something that still allows it to turn. Foam basically, or
* replace the rod with a compression rod - which is my preference
It's really not a horrible effect, so the customer is opting to do nothing. I can fix it if he wants.