My piano is making a new noise that I don't understand. When I play the fifth G from the left (G5?) there is a sustained ring of the same frequency but it doesn't seem to be the strings producing it. The damper is working, and dampening the strings with my finger has no effect on this ring. No other notes produce the sound, and no other G's seem to do it. It is as if there is something resonating somewhere, and it is a new sound that just appeared today out of nowhere. What is causing this???

The piano is either producing an unwanted overtone or there is something else in the room that vibrates at that frequency.

Too hard to tell from here. Could be anything. I had a buzzing come from the inside of a wall. It was so annoying they hired a contractor who found an old unused electrical box left during a renovation.

Any ideas how to go about troubleshooting?

It does seem to be coming from the piano itself. We have had the piano for a few months and this sound just appeared today out of nowhere. It sounds metallic, like the plate maybe. But if I try to dampen the plate it doesn't affect it.

The best way to troubleshoot it is to get a good technician to listen to it.

OK, I figured it out! I'm an idiot. The lowest key had a stuck damper. Thanks all for your help though. My apologies.

I was just going to suggest before I read you last line to place your hands on all of the strings, a few at a time until you eliminated each one as a possible cause but, you found the problem, good job.