Anyone know the likely cause of this sound ?

[ted]

Very occasionally, my piano emits the strange whistle in the attached recording. It is not an artifact of recording and not a sympathetic vibration outside the instrument. It occurs, I suppose, about once a fortnight, usually during very active passages involving both pedals, but try as I might I cannot generate it deliberately. It isn't a major issue, I am just curious, and if somebody knows a likely cause I shall ask my tuner about it next time.

[Bob]

I'm not sure what to listen for.  Is it reverb or resonance with something?

[ted]

About three seconds into it there is a whistling sound, Bob. Yes, it could indeed be a peculiar resonance within the piano itself, but which part is making it and why so infrequently ? That is the puzzle.

[Bob]

Is it possible to recreate it if you play the same music?

Then I was thinking you could dampen string to narrow it down if it's strings.

[ted]

Curiously not Bob. I have tried unsuccessfully to generate it in all sorts of ways. As I say, it happens so seldom it doesn’t affect my music, so I’m not overly concerned. My present conjecture is that it is partly induced by ambient temperature in much the same way as a singing tap, but it is peculiar in that only very specific circumstances initiate it.

[Bob]

I was thinking of that.  You'd have to have to know what was played along with the temp and humidity.