Discordant middle string.
Instruments- discuss specific features, luthiers, instrument problems & questions
@gennaro, before we can really help you, we need to figure out what exactly is up with that middle string. If it's not out of tune, and it's not buzzing on a fret, what could it be?
I have one idea. I had a dulcimer that had a minor problem with one string. It would kind of ring out louder than the others with a very faint buzz. The problem was not the frets, but something else a repair guy at a guitar shop diagnosed right away: the slot in the bridge and/or nut was too flat, allowing the string to vibrate.
Basically, as the string goes through the groove or slot in the bridge and nut, you want it to sit on a single point in the wood or bone or whatever the thing is made of. Is that slot is too flat, it allows the string to vibrate and kind of ring out louder than the other strings and make a noise that is not exactly a buzz, but something less obvious but still annoying. This happens more often when the nut and/bridge are made of wood rather than a harder substance like bone.
The guy who fixed my dulcimer that had this problem did so in about 5 minutes. If you have a really small file to get into those grooves, you might just run it at an angle on either side of that groove in the nut and bridge, trying to create a single point on which the string will sit. If you don't have such a tool, any guitar shop should be able to do this.
I don't know for sure that this is your problem, but the difficulty you have describing it sounds really familiar, and this is my best guess.