To any tech out there: I recently had my acoustic grand piano maintenanced. One particular problem was the 2nd A and 2nd F# below middle C. These notes sounded unfocused with a strange buzz and really sounded underwater and noticeably different than the adjacent bass notes. My tech worked on them for quite some time and was having no luck. Then he tried this: On these two bi-chord notes he twisted the steel strings inside the copper wound wrap. This improved things quite substantially. Can anyone tell me how this works. Is this is a normal part of the acoustic technician's bag of tricks? He didn't explain and I didn't ask. Any comments are appreciated.
Your piano tuner did the right thing to try everything else first. Suppose there were loose hammer heads and this wasn't noticed until after the strings were twisted and things didn't improve? Once the correct repair was done, the tone on those strings might not match the others even if the twist was taken out, not to mention the instability of those strings for a few weeks. I hope he mentioned that these strings will drop in pitch a tad and need to be retuned in a couple weeks.
Btw, you didn't mention the make, size or age of the piano. I would only expect this problem, and twisting strings as an appropriate repair, on an older piano.