"could the "wooo" sound you hear being the natural resonance of the strings? For example, if you play a G on the D-string, you should(will) hear a sympathetic vibration of the G-string"
Yes, but the sound is available at any note, any position. At some points your hear it better, resonance enhances the effect. I have improved on intonation a lot by just trying to focus on this soound. In fact, I feel that this helps me to develop my relative sense of pitch further. (I have no perfect-pitch, but after playing for a few minutes my ears are tuned up!)
More about the temperature:
I'm not sure what the effect of humidity & temperature has on my strings (gut, eudoxa) but I can relate to the fact that this is just something that personal for me.
About the guitar:
Its a marvelous instrument. I use gold-plated strings (I have very agressive sweat! and price/quality/durability rate is the best of any!) , and when they are fresh and tuned to pitch its a great treat for the ears. Depending on your tuning method the G-chord is one of my favorites!
What other aspects of the violintone could be considered for ear-focus techniques? I would like to exchange some ideas on this subject besides the 'woo'-sound!