Official Luthiers Forum!

So When I fret an E note on the A string (7th fret) and play the open G string simultaneously I get this crazy zinging sound that disappears if I touch behind the 7th fret on the A string at the same time, removing the vibration behind the fretted note. Obviously the G note is causing some kind of sympathetic vibration with the A string behind my fretted note, but I've never seen this before. What could cause that, a too low fret behind, kind of like the reverse of when you press down on a low fret?