I have been party to the use of a Stethoscope to assist with identifying a lower noise being present when a Platter Spindle Bearing Modification has been carried out.
Comparing the noise floor between a Unmodified/Modified same model TT has shown substantial differences in noise floor, especially when Unmodified is not with a known service history for the Bearing Assembly.
When checking for the presence of noise related to a Mechanical Operation, my experiences are showing the following:
No Service History - Serviced - Modified, are the order that has shown quite large steps between each other in relation to the perception of noise floor being detected through the Stethoscope.
I have no knowledge of a Stethoscope being used for checking the influence of a Kinetic Energy being transferred and a poor decoupling method being adopted.