GOOD LORD! This thread is longer than the Brooklyn bridge. I’m not going to sit here and read every post. But I did scan the answers and my impression is your shooting in the dark. Here’s my experience with buzzes.
1) Make sure it’s the harp and not something in the room. Move the harp to a different room and try it. If the buzz is still there, it’s the harp.
2) Make the buzz happen-I think you said it was D natural, which octave I don’t remember- and while you are playing the note over and over again move each of the other pedals slowly through all of their positions. If the buzz gets better or worse when a certain pedal is in a certain position, then it’s probably something inside the action OR one of the rods inside the column. You will then have to try touching linkage inside the action with a thin screwdriver to see if touching one link or another stops the buzz.
3) Make the buzz happen as mentioned in 2 above and press your thumb against the side of the nut that the string goes over. Also, if the buzz is happening on natural or sharp, take a larger screwdriver with a flat blade, put it between the back of the disc in question and the action plate and twist the handle, which will force the disc forward and the spindle to which it is attached hard against the front action plate. If that stops the buzz, then the backplate screw for that disc has to be tightened a little. If the buzz stops when you press against the string nut, then tightening the two screws that hold it in place, OR putting a small piece of paper towel or cloth between the string and the nut may stop the buzz. You can also try wrapping the groove in the nut with thread to pad the string a little.
I’ll add to this later. Have to go now.