An original method that works to reduce motor noise is to use
rubber-impregnated belt material that connects the four mounts on the
bottom.  The strap is connected to the mount on each end and the bolt
to the motor goes through the strap from underneath.  The motor is
'suspended' in air by the straps.  Player Piano Co. used to sell these
rubberized straps but I'm not sure if they do now.

Heavy duty coiled springs, approximately 1" tall, were also used for
the mounts.

A new method is to use the modern vibration control mounts available
through Grainger or McMaster-Carr.  These rubber mounts are convenient,
but they tend to twist with substantial belt tension so the motor is
not parallel with the drive pulley.  The larger the diameter of the
rubber, the less twist, but they don't look very original.

They all accomplish the same effect; to 'separate' the motor from the
piano case.  Rubber is probably the best material for this, if it can
be mounted in a way to keep the motor exactly parallel with the drive
pulley.

Brian Smith

 [ Use the minimum belt tension, and inspect the drive belt.  Cheap
 [ automobile V-belts often have bumps which add considerable noise.
 [ -- Robbie