I don't think I saw any mention of verifying that the shift linkage bracket is clearing the bodywork everywhere. Kind of hard to do all around by eye from underneath, maybe work a sheet of heavy cardboard all around it to check clearances.
I had a similar "mystery buzz" occurrence on my tii, and maybe something similar is happening on your carby. It turned out that the throttle body brace that doubles as a starter brace had broken and the TB had worked a bit loose as a result. I think vibration caused it all, alongwith a stripped out bolt hole. Anyway, I welded the bracket back together, heli-coiled and tightened all bolts and screws and the noise went away. It was funny that it didn't really resonate at any particular engine speed, but it convinced me that that brace is pretty important on tii's.