Please go here to see photos of two "steamboat" pump motors (grand
and upright) properly mounted:  http://www.flickr.com/photos/goatwood 

A little farther down on the page is shown a rear view of the grand
motor with its mounting block as screwed to the bottom of the pump
cabinet.  A piece of soft heavy felt goes between the motor platform
and the mounting block, glued to the latter.  Please note that on the
pulley side, most of the motor's weight is supported by the two drive
belts.  On the opposite side, a leather strap supports the motor
platform.  Additionally on the pulley side, a small adjustable
pull-down strap balances the motor's weight and limits the up-and down
movement of the motor during its operation.

Obviously, if any part of the motor platform assembly directly contacts
the bottom of the upright piano, the bottom will act as a soundboard,
greatly amplifying the AC hum of the motor.  Properly mounted, these
motors, including the 1750 RPM version used with the later steamboat
pumps, are incredibly quiet in their operation.

Jeffrey R. Wood