From here, it sounds like a sympathetic vibration of some sort -- not sure why it only occurs when those two notes are played simultaneously. Regardless, start with the last thing you did to it (presumably installing the Anthem). Are all the wires and the volume attenuator securely affixed? How about the endpin jack and the jack cover?

If this doesn't reveal the problem, start at the top and methodically go through the instrument: tuners screws and retaining nuts, truss cover plate, truss rod, saddle, string ball ends, bridge pins, etc. Isolate each variable as much as you can. Something will show up eventually!