One possibility is that you have a so-called “wolf tone” on open G (~196 hz).
What is wolf tone? Well, every guitar has a number of natural resonance frequencies. The two strongest are typically somewhere between F to G on the lower E string, and then also somewhere on the G string. These two resonances relate to the natural vibration frequency of the body chamber itself (for the lower one), and the soundboard alone (for the upper one).
If one of these natural resonances corresponds closely to a note you want to play, then the guitar will acoustically feed back with itself while playing that note. On the lower resonance, this comes across as the so-called “thuddy G” (or F# if it’s on F#). The note sounds louder, but quickly plays itself out and so sustains much less than the other notes on that string. “Thud.”
On the higher resonance, it can sound like a more traditional feedback scenario, with that note playing much louder than the others. That’s the obnoxious sound. The guitar is indeed acoustically feeding back on itself via the soundboard, generating higher volume.
I’ve read that wolf notes are particularly likely to manifest with high quality instruments (because lower quality ones don’t sustain as well or as clearly, and therefore mask the wolf notes). I also have read that wolf notes are even more problematic on cello (because of the bowing action, as opposed to picking or strumming).
My understanding is that there are a large number of variables that affect the resonance frequencies, and getting it right is part of the magic of lutherie. Right meaning, it’s best if the resonance frequencies fall exactly between two notes (so they are least likely to cause a wolf note). Once the guitar is built, it’s too late to change most of the variables. That said, it’s possible to marginally modify the upper resonance frequency by adding mass to the soundboard. Making the soundboard slightly heavier will reduce the resonance frequency.
I had this issue sounding on the G# (208 hz) of one of my guitars. That guitar is carbon and very lightly built. The resonance made G# on the G string sound very odd. Made playing an open E chord in particular sound weird — it wasn’t quite as noticeable if playing single note lines. The effect was also there, though weaker if G# was played on the D or A strings.
I was able to mitigate the problem enough to not mind by swapping in some brass bridge pins (and hence adding mass). For my guitar, 4 or 5 brass pins seemed to be the sweet spot. If I went to 6 brass pins, the resonance moved all the way down to G and that note got obnoxious.
If you don’t have a set of brass pins laying around, you could try taping 4 or 5 quarters just behind the bridge using some masking tape, to see if that mitigates the problem. If yes then you have your answer on what to do.
Another way to add mass, would be to install a soundhole mag pickup. If you have one laying around unused, that’s another cheap way to test to see if it solves the problem.
I don’t know if this is what is troubling your guitar or not, but on the chance it is, I share this experience with you. Best of luck sorting it out.
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