[pianotech] Sub Harmonic "buzzing" in Treble Intervals

[Jeff Atkisson]

Hi, you all don't really know me - but maybe you have a quick answer 
for me.

It's a Stenway M.
Octave 6 and 7 (mainly)
Play a fourth or a fifth say from E6.
Strange (to me) "buzzing' or humming sound at a lower harmonic (i.e. at 
approximately E5 when playing E6-B6).
Piano is in tune. (does it with single strings)
Only does it it with intervals (not single notes).
Lasts for around a second or two ...
Does it with most of the intervals played in this range (not in high 
treble or below about C6).
It  muddies the sound of  chords and intervals there.

It seems like its the board but I can't seem to stop it by pressing on 
ribs - although I haven't done extensive searching in this regard.

You all know what this is?

Please don't tell be its my ears or my imagination ...

pretty sure I could catch it with a mic if I could post the sound clip.

Thanks

Jeff