Try to change the pitch of the string (from g to f for exemple) and play. Is the problem on the same key?
I think where Marcello is going here is to evaluate what is causing the vibration.
If you tune the string down a step and the harmonic is still sounding on the same key (which also sounds a step down) then you know that it's something in the way that the string/wheel/bridge/nut/key/tangent system is working.
If you tune the string down a step and the harmonic no longer sounds on that key, but possibly now sounds on the next key up, it could be a resonance which is intrinisic in the instrument. Every instrument has resonances - to some extent it's why they work. It's the luthier's job to design ane build an instrument where the resonances are evenly distributed.
If I'm totally off the track, Marcello, I hope you will tell me. ;-) Alden
