So my san-no-ito is extremely buzzy and I cannot for the life of me figure out why. Played on an open note, or a high note, it’s mostly fine, but when I play from 6-8 it becomes so buzzy it’s discordant with everything else. Any thoughts?
Hi Morrigan,
It’s sounds like the neck is in need of kanberi (re-leveling). The surface of the sao gets uneven due to natural warping or, more likely the case, frequent playing. That’s happened to my shamisen as well.
To fix it, you have to use sandpaper (180, gradually increasing grit level to 2000) and very evenly run it back and forth over the sao to slowly remove enough wood to level the sao again. (First the pitted area, and gradually over the whole length of the fretboard so the wood removal is even.)
Evenly removing the wood takes some experience, but an instrument luthier could easily do it, I think. It’s quite easy, just takes a steady, level hand.
Extra info: The sao isn’t actually completely level. It’s actually supposed to slope down 1.5~2mm from position around 6 to 14 (or so) to improve intonation. Some guitars are made the same way, I believe. It’s a little tricky to understand without looking at one, but I think a good luthier should understand what to do. 
Thanks Kyle! Luckily I have all the tools I need to make sure that’s the problem, and fix it if it is. I’ll probably post an update in a few days when I get the time to mess with it.
I’ve had my shamisen for 8 years now and am too afraid to alter it but I’ve been considering doing reworking the soa. My san-no-ito falls into way too many pits in the 12~19 note range. Let us know how any adjustments come along - I’m very interested!