I have in my collection a Serling Welte-Mignon licensee upright which
is giving me some trouble.  The governor pneumatics on the expression
unit make a very audible "groaning" noise which can only be silenced
if I turn down the suction on the pump.

As a result, the louder musical passages will drain the pump and the
overall expression suffers as a result.  The bass pneumatic is worse
than the treble, but they both do this to a certain extent only in
softer musical passages.

If you touch one of these pneumatics when it's making this noise you
can feel it vibrating like a palm sander.  I've tried adjusting the
controller springs and dampening them with felt but it makes no
difference.  I've pulled both controller valves from inside these
pneumatics and can find nothing damaged or out of the ordinary.

My restorer used thin pneumatic cloth throughout the expression unit
including the governor pneumatics.  Should a heavier weight material
have been used?  Any suggestions on what could be causing this would
be greatly appreciated.

Best regards, Bill Meyer - Syracuse, N.Y.