THUD

[John D. Chapman]

Remember THUD?
Here is the history.  I had not seen this piano until last month.
While tuning I noticed that notes 50, 51, 52, and 53 (top four notes in
agraff section,  just below the treble break) had an unpleasant
knock/slap sound, the same percussive knock which is in the background of
all piano tone, but in notes 50-53 it dominates the tone.  These are the
four notes between A440 and the treble break.  The orchestra conductor
asked me if I had noticed those four notes, as did the pianist, so I knew
it wasn't just my imagination. I have checked it out both upstage and back
stage (wood floors), and back at the dealership (concrete floor).
        Here is what we have done so far:
1.  checked hammers for loose glue joint, flange pinning, and screw
tightness,
2.  visually checked the action frame joints between notes #53 & #54,
3.  checked the wippens for sufficient felt at the front to keep the
jack from slamming into the rep lever,
4.  checked key frame bedding, front, back, and glides, with and without
damper pedal, and with and without shift pedal,
5.  pressed down the back key frame with a long screwdriver to see if it
was slapping up against the dag,
6.  played notes with dampers held all the way up by hand and by pedal,
7.  moved the action in and out to see if the tone improved,
8.  switched some hammers from the octave below to see if it could be the
hammers,
9.  switched some whippins from the octave below to see if it coulb be the
whippins,
10.  seated the strings,
11.  applied firm pressure on the bridge cap at the end of the section to
see if there was a loose glue joint between cap and bridge body,
12.  checked for clearance between the bridge and the plate,
13.  checked for loose ribs,
14.  checked for clearance between nose and bell bolts and soundboard,
15.  checked that the bell bolt nut which was one turn past finger tight,
16.  checked for foreign object lodged between soundboard and frame, and
between soundboard and plate,
17.  tightened the end screws of the diagonal plate bar, which could be
turned another half turn to snug,
18.  checked key leads and underlever leads,
19.  checked the key bed with a straightedge front to back.  I found that
the board just beyond the board on which the balance rail glides rest was
high.  Chalked it where it passes under the front to back board of the key
frame which is just under notes 50-53 and found that is was just barely
touching. Sanded key frame there until there was adequate clearance.
Playing the note by pushing up on the whippin from underneath with my 
finger without moving the key still produces the THUD, so I guess that 
eliminates the keyframe anyway.  

None of this has changed the THUD.  
What have I missed?  
Why these four consecutive notes  with no sign of the problem above or
below them? 

John Chapman RPT
Wake Forest University
Winston-Salem NC