I just built a parts-caster and cut my first bone nut. I learned how to cut it on U Tube and I think I did a great job, for my first try. But one string pings in the nut, when bending it. I have tried to smooth the grove out some... It is NOT binding in the nut, ... it makes a ;ping' sound when it slides from left to right in the groove. I have widened it a bit, but the pinging continues.. Is there a cure for this? It only happens with the non-wound D string. I lubed it, but it still pings.. Does anyone know why? Should I just cut another nut? Thank you.