Actually it’s the 3rd octave A and G, not the 4th. And it’s not the room, although that is a good suggestion. I just bought the harp used, so I’m not sure about the age of the wires although they look to be in good shape. When I play those 2 strings while dampening the 6th octave wires up toward the top of the harp, I don’t hear that dissonant vibration so maybe I should explore that more.