I have read on various violin and guitar luthier's pages that they shoot for resonant frequencies that lie somewhere between the notes that we would normally sound, assuming concert pitch. Most classical guitars that I've played have had resonant frequencies between Ab3 (207.652 Hz) and A3 (220.00 Hz) and some have been between G3 (195.998 Hz) and Ab3 (207.652 Hz). Violins would be an octave above that.
Now that's assuming concert pitch, which is what most of us use but if you have a faulty electronic tuner (like el baroda) or cannot (or choose not
) to use a tuner at all, you might land on those frequencies. If you're playing with other instruments that tune to pitches higher or lower than concert pitch, say an oboe or a piano, and your guitar is properly calibrated for concert pitch you can also have this problem. As we all know pianos are not always A=440 and oboists' "reference" pitch can vary even wider. In addition, some orchestras intentionally tune to A=442 or higher and this can cause (or cure) problems like this. However, since most of us play solo and tune with an electronic tuner, we can tune up or down to help alleviate this problem as needed. Also, instead of tuning up or down entire semitones, try a few hertz at a time. Shifting your tuner's reference pitch to A=442 or 443 is an easy way to do this. Tuning forks have a tendency to go down, depending on how well they're kept. I've taken great care of my Wittner A440 and it's somewhere around 438. My guitar does respond slightly differently if I tune it with the tuning fork as opposed to my electronic Peterson VS-1 (with the guitar sweetening curve on).
Since I have slight intonation problems with my G and B strings, it got me thinking how that might also be a way for some of your sounded notes to hit the resonance area. If your fretted notes play sharper than your open string, which is fairly common, this could probably explain why some people have noticed these notes appear on the higher strings while the lower strings are fine.
It's interesting to note how hand made guitars seem to suffer from wolf tones and dead spots more than factory made guitars. You would think that the opposite would be true, that hand made guitars would be less susceptible to this due to the greater time spent in the luthier's hands and their ability to fine-tune the guitar before delivery and that factory mades would be more susceptible due to their relatively short assembly time, lower grades of wood and less one-on-one time with the makers. Perhaps as we build more responsive, lively yet balanced instruments, we are also amplifying some of these undesirable frequencies and that's why we're noticing them in the hand made ones more? That's what makes me even more appreciative and respectful when I find a guitar that doesn't have any noticeable problem areas!