Progress! Scott, or anyone...
I got one of the knobs off. Removing them is not such a big deal as I thought. It turns out the brass insert in the knobs is threaded. They are screwed onto the rods AND soldered. Sheesh! The hole in the top metal end plate of the plunger is indeed just a little bigger than the rod... not much more than .010". Photo: http://www.n1bug.com/cavity2.jpg If there originally was an insulated insert I suspect it was a special item for this application (or at least not hardware store variety). But if there wasn't one then I'm left with the original mystery of why it didn't have this noise problem until fairly recently. So what now? I think I could solder the rod to the top of the plunger without dislodging the whole end plate. Is that a bad idea? Better ideas? Alternatively, I think I could just squeeze some .005" PTFE sheet in that gap, but there isn't enough clearance to allow overlapping the ends at all. Also it might not stay put or hold up well with time and vibration. I still can see no other possible source of the noise/desense in this cavity. Paul N1BUG
