For Christmas my now wife bought me my first guitar, a Sigma SD18CE. I really like the sound and and playability of the guitar. However when I was playing Barre chords I was noticing a odd vibration sound when on the 7th fret. I thought it was because I was relatively new and it would work out. Then I started working on a song that required a capo on the 7th fret. I found that I was getting the same vibration sound and it was coming from the G string.

I had already replaced the strings between the times I noticed it so I was sure that wasn't it. After a while I gave up and took the guitar to my local dealer. One of the guys there that is also my instructor, checked several things. Replaced the strings, moved the wires for the electrics and nothing changed. He then looked at the string behind the capo and discovered that the Low E string was resonating. He described it as the voice of my guitar have the voice of D and there was nothing that could be done about it. It just happens to be the way this guitar turned out.

My dealer was very good about it and exchanged my guitar for another one he had in stock. I checked several things out and haven't found any issues with the replacement guitar.
So have you ever come across any similar type resonance or "voicings" that didn't work out or a voicing that enhanced something you were looking for in a guitar?
"Wolf tones" are fairly common in lightly built acoustics. They correspond to one of the major resonance frequencies of the instrument, and can either enhance or kill a particular note - I've seen both. In dreads they often occur as a "boomy" enhancement of the low F# or G. Your experience might have been a related phenomenon, or maybe some fret unevenness on the low frets causing a rattle.