Hey,
I think i have a similar problem with the banjo sound. It sounds a bit like a 'static' click. This also accurs sometimes with some other reso and acoustic sounds. Sometimes it looks that the tone knob on the guitar does affect this popping sound, but i'm not sure. I'm also not sure if this is a firmware issue or perhaps a output volume issue. Anyway, i don't have a fix jet. Maybe somebody else?