does this help? One of the banjos I built a while back had a similar problem which I eventually traced to the planetary tuner on the first string which seemed to have something loose inside when removed from the banjo and shaken. As these aren't normally openable I replaced it and the buzz was gone. Also - check the neck alignment especially where it joins the pot as it is possible for the first or fifth string to catch on the edge of the tension ring if its not put on exactly in the right position. Good hunting. I would appreciate it if you do find the cause as I am always looking for problem solvers
keep picking

Pete