Check the motor mounts.  The motor slides on two rods, with a
collar-and-spring arrangement to provide tension for the belt.
The counter force pushes the ends of the rods against the center
beam of the piano.  The way the molded rubber motor mounts are
made, there isn't much rubber between the ends of the rods and
the beam to absorb the vibration of the motor.

Put some thick felt washers or pieces of heavy rubber tubing on
the rods between the collars and the inside of the rubber motor
mounts to keep the end of the rod away from the beam, or, if
there's room, put some additional padding between the motor mount
and the beam.

Bill Luecht