Look at this as a possible problem. This has happened to me in the past. Make sure none of the loose ends of the strings (bridge side) are touching the soundboard. Don't limit youself to only the G string because the open G note may be causing sympathetic vibrations on any of the string. To put it another way, make sure when you tie on the string, you don't have the free end touching (anything) if you can help it. Certainly not the soundboard.