@thor @freemo On a piano those would be B-C and E-G in frequency. So there's something like a fifth chord occurring? Those are higher pitches to so maybe a guitar is picking them up and it's causing a resonance through the bridge? I believe the second and third strings are tuned to 5ths and the same with the fifth and sixth strings.

With stringed instruments many odd things happen. Trying to tune my guitar next to a piano would make it very difficult even if I muted the strings with my palm. The resonance makes other stringed instruments close to it vibrate at the same frequencies.

It seems like it is causing a set of 5th chords which is pretty cool even if it's a malfunction.