It has taken longer to get back to this than I had anticipated.
I tried applying some upward pressure on the sound board just under the bridge end with the heel of my hand since it was in an open space. I could change the phantom tone a little but it didn't help it any.
I have tried all of the suggestions listed so far but nothing has caused the tone to go away. I have made another video of the phantom sound, but it doesn't come out as strong in the video as it does in real life, this is the link https://youtu.be/OPhmuRAZQX4
Any new thoughts or suggestions would be appreciated.
Just went through the entire thread and listened to both videos....
First of all, this isn't even a question that can be asked until basic maintenance is done. It sounds horrible. This is due to:
1) Piano has severe design limitations.
2) 17 years old and probably hasn't had any real service -- either in new piano prep or in ongoing alignment, regulation, voicing, etc.
So, until those accumulated routine maintenance issues are taken care of, it will be difficult to locate anything else.
What I notice would be traceable to strings un-level (not mated to hammers) and grooved hammers that are hitting the strings with the sides of the grooves.
Once that is dealt with, then you could experiment/search for such things as...
--adding mass to the end of the long bridge
--looking for glue failure of some point of soundboard to rim joint
--crazily enough, I have encountered bass bridge aprons on the cheap Yamaha grands that have the grain of the apron running parallel to the length of the bridge. A split can open up in the apron right near the bridge body that is hard to see (especially under the accumulated dust and gunk) unless you look VERY carefully with good light for that possibility. The fact that the tonal weirdness is growing would be a clue that maybe such a split is opening up further.
Minor nitpick. Terms like G3 or C4 refer to pitches according to tuning aids. Actual notes are properly named as C-28 or E-44, etc.
Hope you get to the bottom of this.