does the G ring a bit when you pick it and it's open? sometimes it is the intonation or the string setup. Check to see how it sits in the nut. I had a problem a little while back because I thought I was SRV and I put on a higher gauge string than my nut (shaddup) could handle. This lead to the "G string buzzin at the nut" thread that still makes me think.. what the hell was I thinking?
I digress
If it's the correct gauge, you could try some graphite in the nut. Be careful and don't get this stuff on the fretboard.. it makes it very slippery :cool: